Question

Lets say I have 4 tables A,B,C,D with records and datetime field in each table.

Currently Im using UNION ALL and ORDER BY Date DESC.

The problem is if a table A has 10 records added today, and table B has 10 records added yesterday then the first 10 results are all from table A.

Is it possible to get records ordered by date, but mix them between all 4 tables?

First record will be the latest added from table A, then the latest added from table B, then C, then D. And cycle them like that?

thanks

Answer 1

Unfortunately, MySQL doesn’t have a ranking function – but you can simulate it with variables. I suggest trying something like the following:

SELECT * FROM 
(select *, @rownum := @rownum + 1 AS rank, 'A' as tbl 
 from (SELECT * FROM (SELECT  A.* FROM A ORDER BY `date` desc) sa) a, 
      (SELECT @rownum := 0) r
 union all
 select *, @rownum2 := @rownum2 + 1 AS rank, 'B' as tbl 
 from (select * from (SELECT  B.* FROM B ORDER BY `date` desc) sb) b, 
      (SELECT @rownum2 := 0) r2
 union all
 select *, @rownum3 := @rownum3 + 1 AS rank, 'C' as tbl 
 from (select * from (SELECT  C.* FROM C ORDER BY `date` desc) sc) c, 
      (SELECT @rownum3 := 0) r3
 union all
 select *, @rownum4 := @rownum4 + 1 AS rank, 'D' as tbl 
 from (select * from (SELECT  D.* FROM D ORDER BY `date` desc) sd) d, 
      (SELECT @rownum4 := 0) r4
) s
order by rank, tbl

Alternatively, try something like:

SELECT * FROM 
(SELECT S.*, 
        @rownum := CASE WHEN @tblvar <> S.tbl THEN 0 ELSE @rownum+1 END AS rank, 
        @tblvar := S.tbl AS tblvar 
 FROM 
 (SELECT * FROM 
  (SELECT  A.*, 'A' as tbl FROM A
   UNION ALL
   SELECT  B.*, 'B' as tbl FROM B
   UNION ALL
   SELECT  C.*, 'C' as tbl FROM C
   UNION ALL
   SELECT  D.*, 'D' as tbl FROM D) U
  ORDER BY tbl, `date` desc) S,
 (SELECT @rownum := 0) R,
 (SELECT @tblvar := '!') T
) Q
order by rank, tbl