Mysql 按日期排序，但过滤结果

Question

假设我有 4 个表 A,B,C,D，每个表中有记录和 datetime 字段。

目前我使用UNION ALL 和ORDER BY Date DESC。

问题是如果表A今天添加了10条记录，而表B昨天添加了10条记录，那么前10条结果都来自表A。

是否可以获得按日期排序的记录，但在所有 4 个表之间混合它们？

第一条记录将是表 A 中最新添加的记录，然后是表 B 中最新添加的记录，然后是 C，然后是 D。然后像这样循环它们？

谢谢

Answer 1

我不确定这是否是您正在寻找的，但我认为您可以使用以下内容：

SELECT
  A.*,
  datediff((select max(`date`) from A), `date`) as d_diff,
  1 as tab
FROM A
UNION ALL
SELECT
  B.*,
  datediff((select max(`date`) from B), `date`) as d_diff,
  2 as tab
FROM B
ORDER BY d_diff, tab, `date` desc

或者这个：

SELECT * FROM (SELECT  A.* FROM A ORDER BY `date` desc limit 5) a
UNION ALL
SELECT * FROM (SELECT  B.* FROM B ORDER BY `date` desc limit 5) b
ORDER BY `date` DESC

根据您的 cmets，这应该可行，但也可能很慢：

SELECT
  A.*,
  (select count(*) from A A1 where A1.dat>A.dat) as i,
  1 as tab
FROM A
UNION ALL
SELECT
  B.*,
  (select count(*) from B B1 where B1.dat>B.dat) as i,
  2 as tab
FROM B
order by i, tab

（可以使用变量更快，但我更喜欢纯 SQL）

Answer 2

不幸的是，MySQL 没有排名功能 - 但您可以使用变量来模拟它。我建议尝试以下方法：

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

或者，尝试类似：

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