Store query result in a variable using in PL/pgSQL

How to assign the result of a query to a variable in PL/pgSQL, the procedural language of PostgreSQL?

I have a function:

CREATE OR REPLACE FUNCTION test(x numeric)
RETURNS character varying AS
$BODY$
DECLARE
name   character varying(255);
begin
 name ='SELECT name FROM test_table where id='||x;
  
 if(name='test')then
  --do somthing
 else
  --do the else part
 end if;
end;
return -- return my process result here
$BODY$
LANGUAGE plpgsql VOLATILE

In the above function I need to store the result of this query:

'SELECT name FROM test_table where id='||x;

to the variable name.

How to process this?

I think you're looking for SELECT select_expressions INTO:

select test_table.name into name from test_table where id = x;

That will pull the name from test_table where id is your function's argument and leave it in the name variable. Don't leave out the table name prefix on test_table.name or you'll get complaints about an ambiguous reference.

To assign a single variable, you can also use plain assignment in a PL/pgSQL code block, with a scalar subquery to the right:

name := (SELECT t.name from test_table t where t.id = x);

Effectively the same as SELECT INTO like @mu already provided, with subtle differences:

• SELECT INTO is slightly faster in my tests on Postgres 14.
  (Plain assignment of a constant, without involving SELECT, is 10x faster, still.)
• SELECT INTO also sets the special variable FOUND, while plain assignment does not. You may want one or the other.
• SELECT INTO can also assign multiple variables at once. See:
  • https://stackoverflow.com/questions/16311634/select-into-with-more-than-one-attribution/16311870#16311870

Notably, this works, too:

name := t.name from test_table t where t.id = x;

A SELECT statement without leading SELECT. But I would not use this hybrid. Better use one of the first two, clearer, documented methods, as @Pavel commented.

The usual pattern is EXISTS(subselect):

BEGIN
  IF EXISTS(SELECT name
              FROM test_table t
             WHERE t.id = x
               AND t.name = 'test')
  THEN
     ---
  ELSE
     ---
  END IF;

This pattern is used in PL/SQL, PL/pgSQL, SQL/PSM, ...

> Create Learning Table:

CREATE TABLE "public"."learning" (
    "api_id" int4 DEFAULT nextval('share_api_api_id_seq'::regclass) NOT NULL,
    "title" varchar(255) COLLATE "default"
);

> Insert Data Learning Table:

INSERT INTO "public"."learning" VALUES ('1', 'Google AI-01');
INSERT INTO "public"."learning" VALUES ('2', 'Google AI-02');
INSERT INTO "public"."learning" VALUES ('3', 'Google AI-01');

> Step: 01

CREATE OR REPLACE FUNCTION get_all (pattern VARCHAR) RETURNS TABLE (
		learn_id INT,
		learn_title VARCHAR
) AS $$
BEGIN
	RETURN QUERY SELECT
		api_id,
		title
	FROM
		learning
	WHERE
		title = pattern ;
END ; $$ LANGUAGE 'plpgsql';

> Step: 02

SELECT * FROM get_all('Google AI-01');

> Step: 03

DROP FUNCTION get_all();

Demo:

Per Executing a Query with a Single-Row Result, use this syntax:

SELECT select_expressions INTO [STRICT] target FROM ...

where target can be a record variable, a row variable, or a comma-separated list of simple variables and record/row fields.

Unlike the SELECT INTO, SELECT select_expressions INTO does not create a table.

In your example, you have a single simple variable name, so the select statement would be:

SELECT test_table.name INTO name FROM test_table WHERE test_table.id = x;

You can use the following example to store a query result in a variable using PL/pgSQL:

 select * into demo from maintenanceactivitytrack ; 
	raise notice'p_maintenanceid:%',demo;