将多个查询合并为一个

Question

Is it possible to join multiple querys in one?

I have 2:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'

and

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Blue'

Now i want both in one query

So is try like this:

SELECT
(
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'
)
(
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff2
FROM Table
WHERE Color = 'Blue'
)

But doenst work :(

Answer 1

You could use a single query with IN operator. This is a short for multiple OR conditions:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color IN ( 'Red', 'Blue' )

If you really insist on (discouraged) having two queries, then use UNION ALL to combine them:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'
UNION ALL -- does not remove duplicates from output
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Blue'

If you want to remove duplicates use UNION instead of UNION ALL .

---

If you need result in different columns as mentioned in a comment then use a CASE statement for that. I really don't see a reason for this though.

SELECT 
  CASE WHEN Color = 'Red' THEN DATEDIFF(CURRENT_DATE, cl.updated_at) END AS Diff,
  CASE WHEN Color = 'Blue' THEN DATEDIFF(CURRENT_DATE, cl.updated_at) END AS Diff2,
FROM Table
WHERE Color IN ( 'Red', 'Blue' )

And for second approach:

SELECT
  CASE WHEN Color = 'Red'  THEN Diff END AS Diff,
  CASE WHEN Color = 'Blue' THEN Diff END AS Diff2
FROM (
  SELECT Color, DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
  FROM Table
  WHERE Color = 'Red'
  UNION ALL -- does not remove duplicates from output
  SELECT Color, DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
  FROM Table
  WHERE Color = 'Blue'
) t

Answer 2

Isn't it just updating the where in this situation?

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff FROM Table WHERE Color = 'Blue' OR Color = 'red'

And yes you can use an INNER JOIN or UNION, but for this example, that wouldn't be useful.

Answer 3

You can use the UNION operator to Join your two queries. https://www.w3schools.com/sql/sql_union.asp

Answer 4

If your output column is same in all your query then you can use UNION / UNION ALL like below:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'

UNION

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Blue'

Answer 5

If you can live with the values in different rows, then just do:

SELECT Color, DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color IN ('Red', 'Blue');

I added Color to the SELECT so you can distinguish the rows.

If the subqueries return one value, then you are simply missing a comma:

SELECT (SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
        FROM Table
        WHERE Color = 'Red'
       ),
       (SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff2
        FROM Table
        WHERE Color = 'Blue'
       );

If the subqueries return multiple rows, then you need to explain the result set you want.

EDIT:

If you want the values to line up in two different columns, then you can to do something like this:

select max(diff_red) as diff_red, max(diff_blue) as diff_blue
from ((select (@rnr := @rnr + 1) as rn, 
              DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff_red,
              NULL AS Diff_blue
       from t cross join
            (select @rnr := 0) params
       where color = 'Red'
      ) union all
      (select (@rnb := @rnb + 1) as rn, 
              NULL AS Diff_red,
              DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff_blue
       from t cross join
            (select @rnrb:= 0) params
       where color = 'Blue'
      )
     ) rb
group by rn;