How to use a postgres variable in the select clause?

779 Asked by darsh_6738 in SQL Server , Asked on Mar 14, 2023

With MSSQL it's easy, the @ marking the start of all variable names allows the parser to know that it is a variable not a column.

This is useful for things like injecting constant values where a select is providing the input to an insert table when copying from a staging table.

declare @foo varchar(50) = 'bar';
select @foo;
How do you express this for postgres?

Answered by David Edmunds

SQL has no support for postgres variables, this is only possible in procedural languages (in Postgres that would e.g. be PL/pgSQL). The way to to this in plain SQL is to use a CTE (which is also a cross platform solution and not tied to any SQL dialect):

with vars (foo) as (

  values ('bar')

)

select foo 

from vars;

like injecting constant values where a select is providing the input to an insert table when copying from a staging table.

Well you don't need a variable for that:

insert into target_table (c1, c2, c3)

select col_1, col_2, 'some value'

from staging_table;

Your Answer

Answer (1)

Ranjana

In PostgreSQL, you can use variables within a SELECT clause in several ways. One common method is to use a DO block or a procedural language function (PL/pgSQL). Here is an example of each approach:

Using a DO Block

A DO block allows you to write an anonymous code block in PL/pgSQL:

  DO $$DECLARE    my_variable INTEGER := 42;BEGIN    RAISE NOTICE 'The value of my_variable is: %', my_variable;    -- You can perform any SELECT operation here using the variable    PERFORM (SELECT column_name FROM table_name WHERE column_name = my_variable);END $$;

Using a Function

You can create a function to use variables within a SELECT statement:

  CREATE OR REPLACE FUNCTION get_data() RETURNS SETOF your_table AS $$DECLARE    my_variable INTEGER := 42;BEGIN    RETURN QUERY    SELECT *    FROM your_table    WHERE some_column = my_variable;END;$$ LANGUAGE plpgsql;-- Call the functionSELECT * FROM get_data();Using a CTE (Common Table Expression) with a WITH Clause

You can also use a Common Table [removed]CTE) to set a variable and use it in a SELECT statement:

  WITH vars AS (    SELECT 42 AS my_variable)SELECT *FROM your_table, varsWHERE your_table.some_column = vars.my_variable;Using Parameters in psql

If you are using psql, the command-line interface for PostgreSQL, you can set and use variables as follows:

  set my_variable 42SELECT *FROM your_tableWHERE some_column = :my_variable;

Example Queries

Example 1: Using a DO Block

  DO $$DECLARE    my_variable TEXT := 'example_value';BEGIN    PERFORM (SELECT column_name FROM table_name WHERE column_name = my_variable);END $$;

Example 2: Using a Function

  CREATE OR REPLACE FUNCTION get_example_data() RETURNS SETOF your_table AS $$DECLARE    my_variable TEXT := 'example_value';BEGIN    RETURN QUERY    SELECT *    FROM your_table    WHERE column_name = my_variable;END;$$ LANGUAGE plpgsql;SELECT * FROM get_example_data();

Example 3: Using a CTE

  WITH vars AS (    SELECT 'example_value' AS my_variable)SELECT *FROM your_table, varsWHERE your_table.column_name = vars.my_variable;

Example 4: Using Parameters in psql

  set my_variable 'example_value'SELECT *FROM your_tableWHERE column_name = :my_variable;

These examples show different ways to use variables in a SELECT statement in PostgreSQL, depending on the context and your specific requirements.

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