Mysql 子查询别名与存在和联合所有

Question

I have a query like this:

SELECT '__HEADER__' as col1, 'HEADER MESSAGE' as col2 
WHERE EXISTS (SELECT col1, col2,
       FROM table1) long_long_query
UNION ALL 
long_long_query

but seems that doesn't like the alias. I'd like to have the header only if results are present, obviously, and not to copy/paste the long long query twice. Is it possible?

Answer 1

You can't use an alias to refer to the subquery used with EXISTS .

If you want to avoid writing the query twice, you can use a Common Table Expression in MySQL 8.x:

WITH long_long_query AS (
    SELECT col1, col2
    FROM table1
)
SELECT SELECT '__HEADER__' as col1, 'HEADER MESSAGE' as col2 
FROM DUAL
WHERE EXISTS (SELECT * FROM long_long_query)
UNION
SELECT * FROM long_long_query;

Prior to 8.x you can define a view.

CREATE VIEW long_long_query AS
SELECT col1, col2
FROM table1;

SELECT SELECT '__HEADER__' as col1, 'HEADER MESSAGE' as col2 
FROM DUAL
WHERE EXISTS (SELECT * FROM long_long_query)
UNION
SELECT * FROM long_long_query;