如何在另一个查询中将SQL查询结果用作列？

Question

I got a view that returns weekday. The result will be a column in WHERE clause, something like this:

 SELECT * FROM dvtv_delivery WHERE (SELECT day FROM dvtv_weekday) = 'Y'

For example:

Today is sunday, in dvtv_weekday returns SUN In dvtv_delivery exist the column SUN

Is it possible in Mysql? When I run nothing appears

Using I use down below, works

  SELECT * FROM dvtv_delivery WHERE SUN = 'Y'

Any answer would really be helpful! Sorry my bad English

Answer 1

The result will be a column in WHERE clause

If that's the case (you want to compare a column), that column should be part of the same query.

SELECT
    *
FROM dvtv_delivery
WHERE
    day = 'Y'

Answer 2

Your query, as written will execute, assuming that the subquery returns exactly one row:

SELECT *
FROM dvtv_delivery
WHERE (SELECT day FROM dvtv_weekday) = 'Y'

Such a query is a scalar subquery . It must return one column and at most one row (under most circumstances).

Often, such subqueries are correlated to the outer query. That means that a WHERE clause connects values in the two tables. It is not clear what a correlation clause would look like in this case, but the structure of the query is:

SELECT d.*
FROM dvtv_delivery d
WHERE (SELECT wd.day
       FROM dvtv_weekday wd
       WHERE wd.? = d.?  -- maybe these would be date columns?
      ) = 'Y'

Answer 3

This query:

SELECT day FROM dvtv_weekday

returns a string like 'SUN' and not the name of the column SUN .
So WHERE is equivalent to:

WHERE 'SUN' = 'Y'

which will always return no rows.
You need something like this:

SELECT * FROM dvtv_delivery 
WHERE 
  CASE (SELECT day FROM dvtv_weekday) 
    WHEN 'SUN' THEN SUN
    WHEN 'MON' THEN MON
    WHEN 'TUE' THEN TUE
    WHEN 'WED' THEN WED
    WHEN 'THU' THEN THU
    WHEN 'FRI' THEN FRI
    WHEN 'SAT' THEN SAT
  END = 'Y'

I assume that the table dvtv_delivery has all these columns: SUN, MON, ....