PostgreSQL 9.5.3 Documentation
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# 40.2. Structure of PL/pgSQL

PL/pgSQL is a block-structured language. The complete text of a function definition must be a block. A block is defined as:

    [ <<label>> ]
    [ DECLARE
        declarations ]
    BEGIN
        statements
    END [ label ];

Each declaration and each statement within a block is terminated by a semicolon. A block that appears within another block must have a semicolon after `END`, as shown above; however the final `END` that concludes a function body does not require a semicolon.

> Tip: A common mistake is to write a semicolon immediately after `BEGIN`. This is incorrect and will result in a syntax error.

A `label` is only needed if you want to identify the block for use in an `EXIT` statement, or to qualify the names of the variables declared in the block. If a label is given after `END`, it must match the label at the block's beginning.

All key words are case-insensitive. Identifiers are implicitly converted to lower case unless double-quoted, just as they are in ordinary SQL commands.

Comments work the same way in PL/pgSQL code as in ordinary SQL. A double dash (`--`) starts a comment that extends to the end of the line. A `/*` starts a block comment that extends to the matching occurrence of `*/`. Block comments nest.

Any statement in the statement section of a block can be a subblock. Subblocks can be used for logical grouping or to localize variables to a small group of statements. Variables declared in a subblock mask any similarly-named variables of outer blocks for the duration of the subblock; but you can access the outer variables anyway if you qualify their names with their block's label. For example:

    CREATE FUNCTION somefunc() RETURNS integer AS $$
    << outerblock >>
    DECLARE
        quantity integer := 30;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 30
        quantity := 50;
        --
        -- Create a subblock
        --
        DECLARE
            quantity integer := 80;
        BEGIN
            RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 80
            RAISE NOTICE 'Outer quantity here is %', outerblock.quantity;  -- Prints 50
        END;

        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 50

        RETURN quantity;
    END;
    $$ LANGUAGE plpgsql;

> Note: There is actually a hidden "outer block" surrounding the body of any PL/pgSQL function. This block provides the declarations of the function's parameters (if any), as well as some special variables such as `FOUND` (see Section 40.5.5). The outer block is labeled with the function's name, meaning that parameters and special variables can be qualified with the function's name.

It is important not to confuse the use of `BEGIN`/`END` for grouping statements in PL/pgSQL with the similarly-named SQL commands for transaction control. PL/pgSQL's `BEGIN`/`END` are only for grouping; they do not start or end a transaction. Functions and trigger procedures are always executed within a transaction established by an outer query — they cannot start or commit that transaction, since there would be no context for them to execute in. However, a block containing an `EXCEPTION` clause effectively forms a subtransaction that can be rolled back without affecting the outer transaction. For more about that see Section 40.6.6.

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