Declare local variables in PostgreSQL?

sql

best-practices

sql-variables

plpgsql

byAlex Kataev·Dec 18, 2024

In PostgreSQL, you'll likely be encapsulating local variables in a DO block for spontaneous execution, or confining them within a PL/pgSQL function. You'll be using a DECLARE clause to declare the variable, subsequently followed by its type and an initialization. Here's a brief DO block illustration:

DO $$
DECLARE
    my_var int := 42; --Where's your answer to the universe now?
BEGIN
    -- Use 'my_var' in your SQL statements here
END $$;

In a PL/pgSQL function, the variable declaration is much alike, done at the beginning:

CREATE FUNCTION my_func() RETURNS void AS $$
DECLARE
    my_var int := 42; --The answer to Life, Universe, and Everything 
BEGIN
    -- What will you do with the secret of our existence?
END $$ LANGUAGE plpgsql;

In these examples, a variable by the name my_var of the data type int is defined and initialized with 42, you can modify as per requirements.

Procedural Potato: PL/pgSQL

Hands deep in the data-digging business of PostgreSQL, you'd quickly realize, PL/pgSQL scripting works wonders. The syntax and logic deviate significantly when compared to similar database systems like MS SQL Server, thus, a thorough understanding reap benefits.

Here's a handy PL/pgSQL function template to help you start declaring variables with default values:

my_var data_type [ := | = | DEFAULT ] initial_value;

A special ALIAS FOR syntax present in PL/pgSQL enables creation of short, precise alternate names for parameters and complex evaluations- who doesn't love a good shortcut?

The cherry on top- practice this coding quirk where direct parameter references are made by their ordinal position in the function:

CREATE FUNCTION compute_sum(a int, b int) RETURNS int AS $$
BEGIN
    RETURN $1 + $2; -- returns "These aren't the droids you're looking for".
END $$ LANGUAGE plpgsql;

In this simple addition function, $1 and $2 correspond to the parameters a and b, regardless of the data type. Time-saving like hitting the light speed!

Navigating from MS SQL to PostgreSQL

Stepping from the familiar lanes of MS SQL to PostgreSQL is quite the adventure. In the exciting landscape alterations of procedural clauses and wire protocols, don’t lose sight of these key differences:

MS SQL thrives on the convenience of stored procedures, whipping up several, variable result sets. PostgreSQL, on the other hand, presents functions primarily returning a single result set or a scalar value, all thanks to the limitations of its wire protocol.
An overflowing toolbox of control-of-flow language in MS SQL often requires diligent translation into PL/pgSQL's block-structured format.

Plan your migration routes wisely, anticipating changes in syntax and scripting to realign with PostgreSQL’s unique features and constraints, especially for the older versions 8.4 and 9.0.

Best practices and potential pitfalls

When working with local variables in PostgreSQL, keep best practices in mind and avoid the common mistakes:

Naming conflicts are like identity theft! Avoid ambiguity between variable names and table column names.
Beware the scope of your transactions; a variable defined inside a DO block or function is restricted to the execution scope of that block.
DO remember that a DO block is like an invisible function that cannot return results to the caller, earning its anonymous status.
PostgreSQL doesn't fulfill the divine SQL right of declaring session-level or global variables directly in the SQL environment. Watch your steps!