SELECT语句中的表达式

SELECT 语句是否允许表达式?后端并不重要,我的意思是更多的概念(iso?)立场。

例如,在做一些基本的测试(使用mysql8,但我也不关心某个特定的后端)时,它似乎被允许在除 LIMIT OFFSET 地点:

with tbl (num, str) as (
    select 1, 'a'
) 
SELECT
    (select num from tbl limit 1) select_expr -- select expression
FROM
    (select * from tbl limit 1) table_expr -- table expression
WHERE
    (select null from tbl)
    or 1 = (select num from tbl)
    or (select num from tbl) = (select num from tbl) -- filter expressions, can be lhs or rhs or single subselect
GROUP BY
    num, (select num from tbl) -- aggregate expression
HAVING
    (select num from tbl limit 1)
ORDER BY
    (select num from tbl limit 1)
LIMIT
    (select num from tbl limit 1) -- invalid
OFFSET
    (select num from tbl limit 1) -- invalid