session_sql_prepare_query(++Session, +ParamTemplate, +ResultTemplate, ++SQLQuery, -Cursor)

Session

A session handle

ParamTemplate

Template defining the types of the parameters (structure or [])

ResultTemplate

Template defining the types of results tuple (structure)

SQLQuery

A SQL query in prepared syntax (string)

Cursor

Returned cursor handle

Description

Prepares a SQL query for execution. The query is not actually executed, and a cursor_next_execute/2 needs to be called to execute the SQL query, giving values to any parameters. Then the cursor_*_tuple family of predicates can be used to obtain the results. This facility is only available if the DBMS supports prepared statements, and the SQL query has to be written in the prepared statement syntax of the DBMS. The predicate returns the cursor handle representing this prepared query in Cursor, which can then be used in subsequent library predicates.

The SQL statement must be a query, i.e. it must return results. If the SQL statement is not a query, an error will be raised.

A prepared SQL statement is parsed by the DBMS, so that it could be executed more efficiently. It can also be parameterised, where the parameters represent values to be filled in when the statement is executed. The statement can be executed multiple times with different parameters. The types of the parameters is specified by ParamTemplate, which is a Prolog structure of arity M (where M is the number of parameters for the statement), or the nil atom [] if there are no parameters. See the general description of this library or the manual for a description of the template specification.

The SQL query returns result in tuples of N elements each. Each tuple is mapped to a Prolog structure of arity N. ResultTemplate is a structure of arity N specifying the types of the return data for ECLiPSe. If N does not match the number of elements expected for the query, an error will be raised. See the general description of this library or the manual for a description of the template specification.

The SQL query must be valid for the DBMS to execute. It can contain NULL characters, i.e. it can contain binary data.

Note that some DBMS restricts which SQL statements can be prepared. If an SQL statement cannot be prepared, it can still be executed using session_sql/3.

MySQL specific note: not all SQL statements can be prepared by MySQL. Refer to the MySQL manual for details.

Exceptions

(5) type error

Session is not a valid session handle, or SQLQuery not a string, or ResultTemplate or ParamTemplate not a structure

(dbi_error)

Error from DBMS while preparing SQLQuery.

(dbi_bad_template)

ResultTemplate or ParamTemplate has the wrong arity

Examples

 make_check_overdraft_limit(Session, Cursor) :-
      % note '?' in SQL in the syntax MySQL uses for placeholders. This may be
      % different in other DBMS
      SQL = "select count(id) from accounts where ID = ? \
                 and balance < overdraft",
      session_sql_prepare_query(Session,a(0),c(0),SQL,Cursor).

See Also