Elements of PSQL

Table of Contents

DML Statements with Parameters
Transactions
Module Structure

A procedural extension may contain declarations of local variables and cursors, assignments, conditional statements, loops, statements for raising custom exceptions, error handling and sending messages (events) to client applications. Triggers have access to special context variables, two arrays that store, respectively, the NEW values for all columns during insert and update activity and the OLD values during update and delete work.

Statements that modify metadata (DDL) are not available in PSQL.

DML Statements with Parameters

If DML statements (SELECT, INSERT, UPDATE, DELETE, etc.) in the body of the module (procedure, trigger or block) use parameters, only named parameters can be used and they must “exist” before the statements can use them. They can be made available by being declared either as input or output parameters in the module's header or as local variables, in DECLARE [VARIABLE] statements at the bottom of the header.

When a DML statement with parameters is included in PSQL code, the parameter name must be prefixed by a colon (“:”) in most situations. The colon is optional in statement syntax that is specific to PSQL, such as assignments and conditionals. The colon prefix on parameters is not required when calling stored procedures from within another PSQL module or in DSQL.

Transactions

Stored procedures are executed in the context of the transaction in which they are called. Triggers are executed as an intrinsic part of the operation of the DML statement: thus, their execution is within the same transaction context as the statement itself. Individual transactions are launched for database event triggers.

Statements that start and end transactions are not available in PSQL, but it is possible to run a statement or a block of statements in an autonomous transaction.

Module Structure

Table of Contents

The Module Header
The Module Body

PSQL code modules consist of a header and a body. The DDL statements for defining them are complex statements; that is, the consist of a single statement that encloses blocks of multiple statements. These statements begin with a verb (CREATE, ALTER, DROP, RECREATE, CREATE OR ALTER) and end with the last END statement of the body.

The Module Header

The header provides the module name and defines any parameters and variables that are used in the body. Stored procedures and PSQL blocks may have input and output parameters. Triggers do not have either input or output parameters.

The header of a trigger indicates the database event (insert, update or delete, or a combination) and the phase of operation (BEFORE or AFTER that event) that will cause it to “fire”.

The Module Body

The body of a PSQL module is a block of statements that run in a logical sequence, like a program. A block of statements is contained within a BEGIN and an END statement. The main BEGIN...END block may contain any number of other BEGIN...END blocks, both embedded and sequential. All statements except BEGIN and END are terminated by semicolons (;). No other character is valid for use as a terminator for PSQL statements.