34.3. Command Execution Functions
34.3.2. Retrieving Query Result Information
34.3.3. Retrieving Other Result Information
34.3.4. Escaping Strings for Inclusion in SQL Commands
Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.
34.3.1. Main Functions
PQexec : Submits a command to the server and waits for the result.
```synopsis
PGresult *PQexec(PGconn *conn, const char *command);
```
Returns a `PGresult` pointer or possibly a null
pointer. A non-null pointer will generally be returned except in
out-of-memory conditions or serious errors such as inability to send
the command to the server. The `PQresultStatus` function
should be called to check the return value for any errors (including
the value of a null pointer, in which case it will return
`PGRES_FATAL_ERROR`). Use
`PQerrorMessage` to get more information about such
errors.
The command string can include multiple SQL commands (separated by semicolons). Multiple queries sent in a single PQexec call are processed in a single transaction, unless there are explicit BEGIN/COMMIT commands included in the query string to divide it into multiple transactions. (See Section 53.2.2.1 for more details about how the server handles multi-query strings.) Note however that the returned PGresult structure describes only the result of the last command executed from the string. Should one of the commands fail, processing of the string stops with it and the returned PGresult describes the error condition.
PQexecParams : Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text.
```synopsis
PGresult *PQexecParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
```
`PQexecParams` is like `PQexec`, but offers additional
functionality: parameter values can be specified separately from the command
string proper, and query results can be requested in either text or binary
format. `PQexecParams` is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
The function arguments are:
**`conn`**
: The connection object to send the command through.
**`command`**
: The SQL command string to be executed. If parameters are used,
they are referred to in the command string as `$1`,
`$2`, etc.
**`nParams`**
: The number of parameters supplied; it is the length of the arrays
**`paramTypes[]`**, **`paramValues[]`**,
**`paramLengths[]`**, and **`paramFormats[]`**. (The
array pointers can be `NULL` when **`nParams`**
is zero.)
**`paramTypes[]`**
: Specifies, by OID, the data types to be assigned to the
parameter symbols. If **`paramTypes`** is
`NULL`, or any particular element in the array
is zero, the server infers a data type for the parameter symbol
in the same way it would do for an untyped literal string.
**`paramValues[]`**
: Specifies the actual values of the parameters. A null pointer
in this array means the corresponding parameter is null;
otherwise the pointer points to a zero-terminated text string
(for text format) or binary data in the format expected by the
server (for binary format).
**`paramLengths[]`**
: Specifies the actual data lengths of binary-format parameters.
It is ignored for null parameters and text-format parameters.
The array pointer can be null when there are no binary parameters.
**`paramFormats[]`**
: Specifies whether parameters are text (put a zero in the
array entry for the corresponding parameter) or binary (put
a one in the array entry for the corresponding parameter).
If the array pointer is null then all parameters are presumed
to be text strings.
Values passed in binary format require knowledge of
the internal representation expected by the backend.
For example, integers must be passed in network byte
order. Passing `numeric` values requires
knowledge of the server storage format, as implemented
in
`src/backend/utils/adt/numeric.c::numeric_send()` and
`src/backend/utils/adt/numeric.c::numeric_recv()`.
**`resultFormat`**
: Specify zero to obtain results in text format, or one to obtain
results in binary format. (There is not currently a provision
to obtain different result columns in different formats,
although that is possible in the underlying protocol.)
The primary advantage of PQexecParams over PQexec is that parameter values can be separated from the command string, thus avoiding the need for tedious and error-prone quoting and escaping.
Unlike PQexec, PQexecParams allows at most one SQL command in the given string. (There can be semicolons in it, but not more than one nonempty command.) This is a limitation of the underlying protocol, but has some usefulness as an extra defense against SQL-injection attacks.
Tip
Specifying parameter types via OIDs is tedious, particularly if you prefer not to hard-wire particular OID values into your program. However, you can avoid doing so even in cases where the server by itself cannot determine the type of the parameter, or chooses a different type than you want. In the SQL command text, attach an explicit cast to the parameter symbol to show what data type you will send. For example:
SELECT * FROM mytable WHERE x = $1::bigint;
This forces parameter $1 to be treated as bigint, whereas by default it would be assigned the same type as x. Forcing the parameter type decision, either this way or by specifying a numeric type OID, is strongly recommended when sending parameter values in binary format, because binary format has less redundancy than text format and so there is less chance that the server will detect a type mismatch mistake for you.
PQprepare : Submits a request to create a prepared statement with the given parameters, and waits for completion.
```synopsis
PGresult *PQprepare(PGconn *conn,
const char *stmtName,
const char *query,
int nParams,
const Oid *paramTypes);
```
`PQprepare` creates a prepared statement for later
execution with `PQexecPrepared`. This feature allows
commands to be executed repeatedly without being parsed and
planned each time; see [PREPARE](sql-prepare.md "PREPARE") for details.
`PQprepare` is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
The function creates a prepared statement named
**`stmtName`** from the **`query`** string, which
must contain a single SQL command. **`stmtName`** can be
`""` to create an unnamed statement, in which case any
pre-existing unnamed statement is automatically replaced; otherwise
it is an error if the statement name is already defined in the
current session. If any parameters are used, they are referred
to in the query as `$1`, `$2`, etc.
**`nParams`** is the number of parameters for which types
are pre-specified in the array **`paramTypes[]`**. (The
array pointer can be `NULL` when
**`nParams`** is zero.) **`paramTypes[]`**
specifies, by OID, the data types to be assigned to the parameter
symbols. If **`paramTypes`** is `NULL`,
or any particular element in the array is zero, the server assigns
a data type to the parameter symbol in the same way it would do
for an untyped literal string. Also, the query can use parameter
symbols with numbers higher than **`nParams`**; data types
will be inferred for these symbols as well. (See
`PQdescribePrepared` for a means to find out
what data types were inferred.)
As with `PQexec`, the result is normally a
`PGresult` object whose contents indicate
server-side success or failure. A null result indicates
out-of-memory or inability to send the command at all. Use
`PQerrorMessage` to get more information about
such errors.
Prepared statements for use with PQexecPrepared can also be created by executing SQL PREPARE statements. Also, although there is no libpq function for deleting a prepared statement, the SQL DEALLOCATE statement can be used for that purpose.
PQexecPrepared : Sends a request to execute a prepared statement with given parameters, and waits for the result.
```synopsis
PGresult *PQexecPrepared(PGconn *conn,
const char *stmtName,
int nParams,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
```
`PQexecPrepared` is like `PQexecParams`,
but the command to be executed is specified by naming a
previously-prepared statement, instead of giving a query string.
This feature allows commands that will be used repeatedly to be
parsed and planned just once, rather than each time they are
executed. The statement must have been prepared previously in
the current session. `PQexecPrepared` is supported
only in protocol 3.0 and later connections; it will fail when
using protocol 2.0.
The parameters are identical to `PQexecParams`, except that the
name of a prepared statement is given instead of a query string, and the
**`paramTypes[]`** parameter is not present (it is not needed since
the prepared statement's parameter types were determined when it was created).
PQdescribePrepared : Submits a request to obtain information about the specified prepared statement, and waits for completion.
```synopsis
PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);
```
`PQdescribePrepared` allows an application to obtain
information about a previously prepared statement.
`PQdescribePrepared` is supported only in protocol 3.0
and later connections; it will fail when using protocol 2.0.
**`stmtName`** can be `""` or `NULL` to reference
the unnamed statement, otherwise it must be the name of an existing
prepared statement. On success, a `PGresult` with
status `PGRES_COMMAND_OK` is returned. The
functions `PQnparams` and
`PQparamtype` can be applied to this
`PGresult` to obtain information about the parameters
of the prepared statement, and the functions
`PQnfields`, `PQfname`,
`PQftype`, etc provide information about the
result columns (if any) of the statement.
PQdescribePortal : Submits a request to obtain information about the specified portal, and waits for completion.
```synopsis
PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
```
`PQdescribePortal` allows an application to obtain
information about a previously created portal.
(libpq does not provide any direct access to
portals, but you can use this function to inspect the properties
of a cursor created with a `DECLARE CURSOR` SQL command.)
`PQdescribePortal` is supported only in protocol 3.0
and later connections; it will fail when using protocol 2.0.
**`portalName`** can be `""` or `NULL` to reference
the unnamed portal, otherwise it must be the name of an existing
portal. On success, a `PGresult` with status
`PGRES_COMMAND_OK` is returned. The functions
`PQnfields`, `PQfname`,
`PQftype`, etc can be applied to the
`PGresult` to obtain information about the result
columns (if any) of the portal.
The PGresult structure encapsulates the result returned by the server. libpq application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future.
PQresultStatus : Returns the result status of the command.
```synopsis
ExecStatusType PQresultStatus(const PGresult *res);
```
`PQresultStatus` can return one of the following values:
`PGRES_EMPTY_QUERY`
: The string sent to the server was empty.
`PGRES_COMMAND_OK`
: Successful completion of a command returning no data.
`PGRES_TUPLES_OK`
: Successful completion of a command returning data (such as
a `SELECT` or `SHOW`).
`PGRES_COPY_OUT`
: Copy Out (from server) data transfer started.
`PGRES_COPY_IN`
: Copy In (to server) data transfer started.
`PGRES_BAD_RESPONSE`
: The server's response was not understood.
`PGRES_NONFATAL_ERROR`
: A nonfatal error (a notice or warning) occurred.
`PGRES_FATAL_ERROR`
: A fatal error occurred.
`PGRES_COPY_BOTH`
: Copy In/Out (to and from server) data transfer started. This
feature is currently used only for streaming replication,
so this status should not occur in ordinary applications.
`PGRES_SINGLE_TUPLE`
: The `PGresult` contains a single result tuple
from the current command. This status occurs only when
single-row mode has been selected for the query
(see [Section 34.5](libpq-single-row-mode.md "34.5. Retrieving Query Results Row-By-Row")).
If the result status is `PGRES_TUPLES_OK` or
`PGRES_SINGLE_TUPLE`, then
the functions described below can be used to retrieve the rows
returned by the query. Note that a `SELECT`
command that happens to retrieve zero rows still shows
`PGRES_TUPLES_OK`.
`PGRES_COMMAND_OK` is for commands that can never
return rows (`INSERT` or `UPDATE`
without a `RETURNING` clause,
etc.). A response of `PGRES_EMPTY_QUERY` might
indicate a bug in the client software.
A result of status `PGRES_NONFATAL_ERROR` will
never be returned directly by `PQexec` or other
query execution functions; results of this kind are instead passed
to the notice processor (see [Section 34.12](libpq-notice-processing.md "34.12. Notice Processing")).
PQresStatus : Converts the enumerated type returned by PQresultStatus into a string constant describing the status code. The caller should not free the result.
```synopsis
char *PQresStatus(ExecStatusType status);
```
PQresultErrorMessage : Returns the error message associated with the command, or an empty string if there was no error.
```synopsis
char *PQresultErrorMessage(const PGresult *res);
```
If there was an error, the returned string will include a trailing
newline. The caller should not free the result directly. It will
be freed when the associated `PGresult` handle is
passed to `PQclear`.
Immediately following a `PQexec` or
`PQgetResult` call,
`PQerrorMessage` (on the connection) will return
the same string as `PQresultErrorMessage` (on
the result). However, a `PGresult` will
retain its error message until destroyed, whereas the connection's
error message will change when subsequent operations are done.
Use `PQresultErrorMessage` when you want to
know the status associated with a particular
`PGresult`; use
`PQerrorMessage` when you want to know the
status from the latest operation on the connection.
PQresultVerboseErrorMessage : Returns a reformatted version of the error message associated with a PGresult object.
```synopsis
char *PQresultVerboseErrorMessage(const PGresult *res,
PGVerbosity verbosity,
PGContextVisibility show_context);
```
In some situations a client might wish to obtain a more detailed
version of a previously-reported error.
`PQresultVerboseErrorMessage` addresses this need
by computing the message that would have been produced
by `PQresultErrorMessage` if the specified
verbosity settings had been in effect for the connection when the
given `PGresult` was generated. If
the `PGresult` is not an error result,
“PGresult is not an error result” is reported instead.
The returned string includes a trailing newline.
Unlike most other functions for extracting data from
a `PGresult`, the result of this function is a freshly
allocated string. The caller must free it
using `PQfreemem()` when the string is no longer needed.
A NULL return is possible if there is insufficient memory.
PQresultErrorField : Returns an individual field of an error report.
```synopsis
char *PQresultErrorField(const PGresult *res, int fieldcode);
```
**`fieldcode`** is an error field identifier; see the symbols
listed below. `NULL` is returned if the
`PGresult` is not an error or warning result,
or does not include the specified field. Field values will normally
not include a trailing newline. The caller should not free the
result directly. It will be freed when the
associated `PGresult` handle is passed to
`PQclear`.
The following field codes are available:
`PG_DIAG_SEVERITY`
: The severity; the field contents are `ERROR`,
`FATAL`, or `PANIC` (in an error message),
or `WARNING`, `NOTICE`, `DEBUG`,
`INFO`, or `LOG` (in a notice message), or
a localized translation of one of these. Always present.
`PG_DIAG_SEVERITY_NONLOCALIZED`
: The severity; the field contents are `ERROR`,
`FATAL`, or `PANIC` (in an error message),
or `WARNING`, `NOTICE`, `DEBUG`,
`INFO`, or `LOG` (in a notice message).
This is identical to the `PG_DIAG_SEVERITY` field except
that the contents are never localized. This is present only in
reports generated by PostgreSQL versions 9.6
and later.
`PG_DIAG_SQLSTATE`
: The SQLSTATE code for the error. The SQLSTATE code identifies
the type of error that has occurred; it can be used by
front-end applications to perform specific operations (such
as error handling) in response to a particular database error.
For a list of the possible SQLSTATE codes, see [Appendix A](errcodes-appendix.md "Appendix A. PostgreSQL Error Codes"). This field is not localizable,
and is always present.
`PG_DIAG_MESSAGE_PRIMARY`
: The primary human-readable error message (typically one line).
Always present.
`PG_DIAG_MESSAGE_DETAIL`
: Detail: an optional secondary error message carrying more
detail about the problem. Might run to multiple lines.
`PG_DIAG_MESSAGE_HINT`
: Hint: an optional suggestion what to do about the problem.
This is intended to differ from detail in that it offers advice
(potentially inappropriate) rather than hard facts. Might
run to multiple lines.
`PG_DIAG_STATEMENT_POSITION`
: A string containing a decimal integer indicating an error cursor
position as an index into the original statement string. The
first character has index 1, and positions are measured in
characters not bytes.
`PG_DIAG_INTERNAL_POSITION`
: This is defined the same as the
`PG_DIAG_STATEMENT_POSITION` field, but it is used
when the cursor position refers to an internally generated
command rather than the one submitted by the client. The
`PG_DIAG_INTERNAL_QUERY` field will always appear when
this field appears.
`PG_DIAG_INTERNAL_QUERY`
: The text of a failed internally-generated command. This could
be, for example, a SQL query issued by a PL/pgSQL function.
`PG_DIAG_CONTEXT`
: An indication of the context in which the error occurred.
Presently this includes a call stack traceback of active
procedural language functions and internally-generated queries.
The trace is one entry per line, most recent first.
`PG_DIAG_SCHEMA_NAME`
: If the error was associated with a specific database object,
the name of the schema containing that object, if any.
`PG_DIAG_TABLE_NAME`
: If the error was associated with a specific table, the name of the
table. (Refer to the schema name field for the name of the
table's schema.)
`PG_DIAG_COLUMN_NAME`
: If the error was associated with a specific table column, the name
of the column. (Refer to the schema and table name fields to
identify the table.)
`PG_DIAG_DATATYPE_NAME`
: If the error was associated with a specific data type, the name of
the data type. (Refer to the schema name field for the name of
the data type's schema.)
`PG_DIAG_CONSTRAINT_NAME`
: If the error was associated with a specific constraint, the name
of the constraint. Refer to fields listed above for the
associated table or domain. (For this purpose, indexes are
treated as constraints, even if they weren't created with
constraint syntax.)
`PG_DIAG_SOURCE_FILE`
: The file name of the source-code location where the error was
reported.
`PG_DIAG_SOURCE_LINE`
: The line number of the source-code location where the error
was reported.
`PG_DIAG_SOURCE_FUNCTION`
: The name of the source-code function reporting the error.
### Note
The fields for schema name, table name, column name, data type name,
and constraint name are supplied only for a limited number of error
types; see [Appendix A](errcodes-appendix.md "Appendix A. PostgreSQL Error Codes"). Do not assume that
the presence of any of these fields guarantees the presence of
another field. Core error sources observe the interrelationships
noted above, but user-defined functions may use these fields in other
ways. In the same vein, do not assume that these fields denote
contemporary objects in the current database.
The client is responsible for formatting displayed information to meet
its needs; in particular it should break long lines as needed.
Newline characters appearing in the error message fields should be
treated as paragraph breaks, not line breaks.
Errors generated internally by libpq will
have severity and primary message, but typically no other fields.
Errors returned by a pre-3.0-protocol server will include severity and
primary message, and sometimes a detail message, but no other fields.
Note that error fields are only available from
`PGresult` objects, not
`PGconn` objects; there is no
`PQerrorField` function.
PQclear : Frees the storage associated with a PGresult. Every command result should be freed via PQclear when it is no longer needed.
```synopsis
void PQclear(PGresult *res);
```
You can keep a `PGresult` object around for
as long as you need it; it does not go away when you issue a new
command, nor even if you close the connection. To get rid of it,
you must call `PQclear`. Failure to do this
will result in memory leaks in your application.
34.3.2. Retrieving Query Result Information
These functions are used to extract information from a PGresult object that represents a successful query result (that is, one that has status PGRES_TUPLES_OK or PGRES_SINGLE_TUPLE). They can also be used to extract information from a successful Describe operation: a Describe's result has all the same column information that actual execution of the query would provide, but it has zero rows. For objects with other status values, these functions will act as though the result has zero rows and zero columns.
PQntuples : Returns the number of rows (tuples) in the query result. (Note that PGresult objects are limited to no more than INT_MAX rows, so an int result is sufficient.)
```synopsis
int PQntuples(const PGresult *res);
```
PQnfields : Returns the number of columns (fields) in each row of the query result.
```synopsis
int PQnfields(const PGresult *res);
```
PQfname : Returns the column name associated with the given column number. Column numbers start at 0. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.
```synopsis
char *PQfname(const PGresult *res,
int column_number);
```
`NULL` is returned if the column number is out of range.
PQfnumber : Returns the column number associated with the given column name.
```synopsis
int PQfnumber(const PGresult *res,
const char *column_name);
```
-1 is returned if the given name does not match any column.
The given name is treated like an identifier in an SQL command,
that is, it is downcased unless double-quoted. For example, given
a query result generated from the SQL command:
```programlisting
SELECT 1 AS FOO, 2 AS "BAR";
```
we would have the results:
```programlisting
PQfname(res, 0) foo
PQfname(res, 1) BAR
PQfnumber(res, "FOO") 0
PQfnumber(res, "foo") 0
PQfnumber(res, "BAR") -1
PQfnumber(res, "\"BAR\"") 1
```
PQftable : Returns the OID of the table from which the given column was fetched. Column numbers start at 0.
```synopsis
Oid PQftable(const PGresult *res,
int column_number);
```
`InvalidOid` is returned if the column number is out of range,
or if the specified column is not a simple reference to a table column,
or when using pre-3.0 protocol.
You can query the system table `pg_class` to determine
exactly which table is referenced.
The type `Oid` and the constant
`InvalidOid` will be defined when you include
the libpq header file. They will both
be some integer type.
PQftablecol : Returns the column number (within its table) of the column making up the specified query result column. Query-result column numbers start at 0, but table columns have nonzero numbers.
```synopsis
int PQftablecol(const PGresult *res,
int column_number);
```
Zero is returned if the column number is out of range, or if the
specified column is not a simple reference to a table column, or
when using pre-3.0 protocol.
PQfformat : Returns the format code indicating the format of the given column. Column numbers start at 0.
```synopsis
int PQfformat(const PGresult *res,
int column_number);
```
Format code zero indicates textual data representation, while format
code one indicates binary representation. (Other codes are reserved
for future definition.)
PQftype : Returns the data type associated with the given column number. The integer returned is the internal OID number of the type. Column numbers start at 0.
```synopsis
Oid PQftype(const PGresult *res,
int column_number);
```
You can query the system table `pg_type` to
obtain the names and properties of the various data types. The
OIDs of the built-in data types are defined
in the file `src/include/catalog/pg_type_d.h`
in the source tree.
PQfmod : Returns the type modifier of the column associated with the given column number. Column numbers start at 0.
```synopsis
int PQfmod(const PGresult *res,
int column_number);
```
The interpretation of modifier values is type-specific; they
typically indicate precision or size limits. The value -1 is
used to indicate “no information available”. Most data
types do not use modifiers, in which case the value is always
-1.
PQfsize : Returns the size in bytes of the column associated with the given column number. Column numbers start at 0.
```synopsis
int PQfsize(const PGresult *res,
int column_number);
```
`PQfsize` returns the space allocated for this column
in a database row, in other words the size of the server's
internal representation of the data type. (Accordingly, it is
not really very useful to clients.) A negative value indicates
the data type is variable-length.
PQbinaryTuples : Returns 1 if the PGresult contains binary data and 0 if it contains text data.
```synopsis
int PQbinaryTuples(const PGresult *res);
```
This function is deprecated (except for its use in connection with
`COPY`), because it is possible for a single
`PGresult` to contain text data in some columns and
binary data in others. `PQfformat` is preferred.
`PQbinaryTuples` returns 1 only if all columns of the
result are binary (format 1).
PQgetvalue : Returns a single field value of one row of a PGresult. Row and column numbers start at 0. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.
```synopsis
char *PQgetvalue(const PGresult *res,
int row_number,
int column_number);
```
For data in text format, the value returned by
`PQgetvalue` is a null-terminated character
string representation of the field value. For data in binary
format, the value is in the binary representation determined by
the data type's `typsend` and `typreceive`
functions. (The value is actually followed by a zero byte in
this case too, but that is not ordinarily useful, since the
value is likely to contain embedded nulls.)
An empty string is returned if the field value is null. See
`PQgetisnull` to distinguish null values from
empty-string values.
The pointer returned by `PQgetvalue` points
to storage that is part of the `PGresult`
structure. One should not modify the data it points to, and one
must explicitly copy the data into other storage if it is to be
used past the lifetime of the `PGresult`
structure itself.
PQgetisnull : Tests a field for a null value. Row and column numbers start at 0.
```synopsis
int PQgetisnull(const PGresult *res,
int row_number,
int column_number);
```
This function returns 1 if the field is null and 0 if it
contains a non-null value. (Note that
`PQgetvalue` will return an empty string,
not a null pointer, for a null field.)
PQgetlength : Returns the actual length of a field value in bytes. Row and column numbers start at 0.
```synopsis
int PQgetlength(const PGresult *res,
int row_number,
int column_number);
```
This is the actual data length for the particular data value,
that is, the size of the object pointed to by
`PQgetvalue`. For text data format this is
the same as `strlen()`. For binary format this is
essential information. Note that one should **not**
rely on `PQfsize` to obtain the actual data
length.
PQnparams : Returns the number of parameters of a prepared statement.
```synopsis
int PQnparams(const PGresult *res);
```
This function is only useful when inspecting the result of
`PQdescribePrepared`. For other types of queries it
will return zero.
PQparamtype : Returns the data type of the indicated statement parameter. Parameter numbers start at 0.
```synopsis
Oid PQparamtype(const PGresult *res, int param_number);
```
This function is only useful when inspecting the result of
`PQdescribePrepared`. For other types of queries it
will return zero.
PQprint : Prints out all the rows and, optionally, the column names to the specified output stream.
```synopsis
void PQprint(FILE *fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
typedef struct
{
pqbool header; /* print output field headings and row count */
pqbool align; /* fill align the fields */
pqbool standard; /* old brain dead format */
pqbool html3; /* output HTML tables */
pqbool expanded; /* expand tables */
pqbool pager; /* use pager for output if needed */
char *fieldSep; /* field separator */
char *tableOpt; /* attributes for HTML table element */
char *caption; /* HTML table caption */
char **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;
```
This function was formerly used by psql
to print query results, but this is no longer the case. Note
that it assumes all the data is in text format.
34.3.3. Retrieving Other Result Information
These functions are used to extract other information from PGresult objects.
PQcmdStatus : Returns the command status tag from the SQL command that generated the PGresult.
```synopsis
char *PQcmdStatus(PGresult *res);
```
Commonly this is just the name of the command, but it might include
additional data such as the number of rows processed. The caller
should not free the result directly. It will be freed when the
associated `PGresult` handle is passed to
`PQclear`.
PQcmdTuples : Returns the number of rows affected by the SQL command.
```synopsis
char *PQcmdTuples(PGresult *res);
```
This function returns a string containing the number of rows
affected by the SQL statement that generated the
`PGresult`. This function can only be used following
the execution of a `SELECT`, `CREATE TABLE AS`,
`INSERT`, `UPDATE`, `DELETE`,
`MOVE`, `FETCH`, or `COPY` statement,
or an `EXECUTE` of a prepared query that contains an
`INSERT`, `UPDATE`, or `DELETE` statement.
If the command that generated the `PGresult` was anything
else, `PQcmdTuples` returns an empty string. The caller
should not free the return value directly. It will be freed when
the associated `PGresult` handle is passed to
`PQclear`.
PQoidValue : Returns the OID of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs, or a EXECUTE of a prepared query containing a suitable INSERT statement. Otherwise, this function returns InvalidOid. This function will also return InvalidOid if the table affected by the INSERT statement does not contain OIDs.
```synopsis
Oid PQoidValue(const PGresult *res);
```
PQoidStatus : This function is deprecated in favor of PQoidValue and is not thread-safe. It returns a string with the OID of the inserted row, while PQoidValue returns the OID value.
```synopsis
char *PQoidStatus(const PGresult *res);
```
34.3.4. Escaping Strings for Inclusion in SQL Commands
PQescapeLiteral : synopsis char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);
`PQescapeLiteral` escapes a string for
use within an SQL command. This is useful when inserting data
values as literal constants in SQL commands. Certain characters
(such as quotes and backslashes) must be escaped to prevent them
from being interpreted specially by the SQL parser.
`PQescapeLiteral` performs this operation.
`PQescapeLiteral` returns an escaped version of the
**`str`** parameter in memory allocated with
`malloc()`. This memory should be freed using
`PQfreemem()` when the result is no longer needed.
A terminating zero byte is not required, and should not be
counted in **`length`**. (If a terminating zero byte is found
before **`length`** bytes are processed,
`PQescapeLiteral` stops at the zero; the behavior is
thus rather like `strncpy`.) The
return string has all special characters replaced so that they can
be properly processed by the PostgreSQL
string literal parser. A terminating zero byte is also added. The
single quotes that must surround PostgreSQL
string literals are included in the result string.
On error, `PQescapeLiteral` returns `NULL` and a suitable
message is stored in the **`conn`** object.
### Tip
It is especially important to do proper escaping when handling
strings that were received from an untrustworthy source.
Otherwise there is a security risk: you are vulnerable to
“SQL injection” attacks wherein unwanted SQL commands are
fed to your database.
Note that it is neither necessary nor correct to do escaping when a data
value is passed as a separate parameter in `PQexecParams` or
its sibling routines.
PQescapeIdentifier : synopsis char *PQescapeIdentifier(PGconn *conn, const char *str, size_t length);
`PQescapeIdentifier` escapes a string for
use as an SQL identifier, such as a table, column, or function name.
This is useful when a user-supplied identifier might contain
special characters that would otherwise not be interpreted as part
of the identifier by the SQL parser, or when the identifier might
contain upper case characters whose case should be preserved.
`PQescapeIdentifier` returns a version of the
**`str`** parameter escaped as an SQL identifier
in memory allocated with `malloc()`. This memory must be
freed using `PQfreemem()` when the result is no longer
needed. A terminating zero byte is not required, and should not be
counted in **`length`**. (If a terminating zero byte is found
before **`length`** bytes are processed,
`PQescapeIdentifier` stops at the zero; the behavior is
thus rather like `strncpy`.) The
return string has all special characters replaced so that it
will be properly processed as an SQL identifier. A terminating zero byte
is also added. The return string will also be surrounded by double
quotes.
On error, `PQescapeIdentifier` returns `NULL` and a suitable
message is stored in the **`conn`** object.
### Tip
As with string literals, to prevent SQL injection attacks,
SQL identifiers must be escaped when they are received from an
untrustworthy source.
PQescapeStringConn : synopsis size_t PQescapeStringConn(PGconn *conn, char *to, const char *from, size_t length, int *error);
`PQescapeStringConn` escapes string literals, much like
`PQescapeLiteral`. Unlike `PQescapeLiteral`,
the caller is responsible for providing an appropriately sized buffer.
Furthermore, `PQescapeStringConn` does not generate the
single quotes that must surround PostgreSQL string
literals; they should be provided in the SQL command that the
result is inserted into. The parameter **`from`** points to
the first character of the string that is to be escaped, and the
**`length`** parameter gives the number of bytes in this
string. A terminating zero byte is not required, and should not be
counted in **`length`**. (If a terminating zero byte is found
before **`length`** bytes are processed,
`PQescapeStringConn` stops at the zero; the behavior is
thus rather like `strncpy`.) **`to`** shall point
to a buffer that is able to hold at least one more byte than twice
the value of **`length`**, otherwise the behavior is undefined.
Behavior is likewise undefined if the **`to`** and
**`from`** strings overlap.
If the **`error`** parameter is not `NULL`, then
`*error` is set to zero on success, nonzero on error.
Presently the only possible error conditions involve invalid multibyte
encoding in the source string. The output string is still generated
on error, but it can be expected that the server will reject it as
malformed. On error, a suitable message is stored in the
**`conn`** object, whether or not **`error`** is `NULL`.
`PQescapeStringConn` returns the number of bytes written
to **`to`**, not including the terminating zero byte.
PQescapeString : PQescapeString is an older, deprecated version of PQescapeStringConn.
```synopsis
size_t PQescapeString (char *to, const char *from, size_t length);
```
The only difference from `PQescapeStringConn` is that
`PQescapeString` does not take `PGconn`
or **`error`** parameters.
Because of this, it cannot adjust its behavior depending on the
connection properties (such as character encoding) and therefore
**it might give the wrong results**. Also, it has no way
to report error conditions.
`PQescapeString` can be used safely in
client programs that work with only one PostgreSQL
connection at a time (in this case it can find out what it needs to
know “behind the scenes”). In other contexts it is a security
hazard and should be avoided in favor of
`PQescapeStringConn`.
PQescapeByteaConn : Escapes binary data for use within an SQL command with the type bytea. As with PQescapeStringConn, this is only used when inserting data directly into an SQL command string.
```synopsis
unsigned char *PQescapeByteaConn(PGconn *conn,
const unsigned char *from,
size_t from_length,
size_t *to_length);
```
Certain byte values must be escaped when used as part of a
`bytea` literal in an SQL statement.
`PQescapeByteaConn` escapes bytes using
either hex encoding or backslash escaping. See [Section 8.4](datatype-binary.md "8.4. Binary Data Types") for more information.
The **`from`** parameter points to the first
byte of the string that is to be escaped, and the
**`from_length`** parameter gives the number of
bytes in this binary string. (A terminating zero byte is
neither necessary nor counted.) The **`to_length`**
parameter points to a variable that will hold the resultant
escaped string length. This result string length includes the terminating
zero byte of the result.
`PQescapeByteaConn` returns an escaped version of the
**`from`** parameter binary string in memory
allocated with `malloc()`. This memory should be freed using
`PQfreemem()` when the result is no longer needed. The
return string has all special characters replaced so that they can
be properly processed by the PostgreSQL
string literal parser, and the `bytea` input function. A
terminating zero byte is also added. The single quotes that must
surround PostgreSQL string literals are
not part of the result string.
On error, a null pointer is returned, and a suitable error message
is stored in the **`conn`** object. Currently, the only
possible error is insufficient memory for the result string.
PQescapeBytea : PQescapeBytea is an older, deprecated version of PQescapeByteaConn.
```synopsis
unsigned char *PQescapeBytea(const unsigned char *from,
size_t from_length,
size_t *to_length);
```
The only difference from `PQescapeByteaConn` is that
`PQescapeBytea` does not take a `PGconn`
parameter. Because of this, `PQescapeBytea` can
only be used safely in client programs that use a single
PostgreSQL connection at a time (in this case
it can find out what it needs to know “behind the
scenes”). It **might give the wrong results** if
used in programs that use multiple database connections (use
`PQescapeByteaConn` in such cases).
PQunescapeBytea : Converts a string representation of binary data into binary data — the reverse of PQescapeBytea. This is needed when retrieving bytea data in text format, but not when retrieving it in binary format.
```synopsis
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
```
The **`from`** parameter points to a string
such as might be returned by `PQgetvalue` when applied
to a `bytea` column. `PQunescapeBytea`
converts this string representation into its binary representation.
It returns a pointer to a buffer allocated with
`malloc()`, or `NULL` on error, and puts the size of
the buffer in **`to_length`**. The result must be
freed using `PQfreemem` when it is no longer needed.
This conversion is not exactly the inverse of
`PQescapeBytea`, because the string is not expected
to be “escaped” when received from `PQgetvalue`.
In particular this means there is no need for string quoting considerations,
and so no need for a `PGconn` parameter.