如何在Postgres中使用DO

Question

I am attempting to get a better understanding of the DO command in postgreSQL 9.1

I have following code block,

DO
$do$
BEGIN
IF 1=1 THEN
SELECT 'foo';
ELSE
SELECT 'bar';
END IF;
END
$do$

However it returns the following error:

[42601] ERROR: query has no destination for result data Hint: If you want to discard the results of a SELECT, use PERFORM instead. Where: PL/pgSQL function "inline_code_block" line 4 at SQL statement

Answer 1

PostgreSQL DO command creates and executes some specific short life function. This function has not any interface, and then it cannot to return any output other then changes data in tables and some debug output.

Your example has more than one issues:

1. Only PostgreSQL table functions can returns some tabular data. But the mechanism is significantly different than MSSQL. PostgreSQL user's should to use RETURN NEXT or RETURN QUERY commands.

    CREATE OR REPLACE FUNCTION foo(a int) RETURNS TABLE(b int, c int) AS $$ BEGIN RETURN QUERY SELECT i, i + 1 FROM generate_series(1,a) g(i); END; $$ LANGUAGE plpgsql; SELECT * FROM foo(10); 

2. DO anonymous functions are not table functions - so no output is allowed.

3. PostgreSQL 9.1 is not supported version, please upgrade

If you have some experience only from MSSQL, then forget it. A concept of stored procedures of PostgreSQL is very similar to Oracle or DB2, and it is significantly different to MSSQL does. T-SQL integrates procedural and SQL constructs to one set. Oracle, PostgreSQL, ... procedural functionality can embedded SQL, but procedural functionality is not integrated to SQL.

Please, read PostgreSQL PLpgSQL documentation for better imagine about this technology.