如何在大查询中联合表

Question

select a,b,c from(SELECT max(modifiedtime) a FROM tabl1 ),
(SELECT max(modifiedtime) b FROM tabl2 ),
(SELECT max(modifiedtime) c FROM tabl3  ) ;

工作正常，但......输出就像.. 行

      a                  b              c    
1   1421906942814      null            null  
2   null               1421906942814   null  
3   null               null            1421906942817

但我想打印像

这样的数据

     a            b              c 
1421906942814     1421906942814     1421906942817

怎么做......

Answer 1

据我所知，你需要使用一个虚拟键（即使它是一个常量）才能在它上面进行JOIN。

这有效：

SELECT t1.value,
       t2.value,
       t3.value
FROM
  (SELECT 1 AS dummykey,
          'max1' AS value) t1
JOIN
  (SELECT 1 AS dummykey,
          'max2' AS value) t2 ON t1.dummykey=t2.dummykey
JOIN
  (SELECT 1 AS dummykey,
          'max3' AS value) t3 ON t1.dummykey=t3.dummykey

Answer 2

试

  SELECT
  (SELECT max(modifiedtime)
   FROM tabl1) a,
  (SELECT max(modifiedtime)
   FROM tabl2) b,
  (SELECT max(modifiedtime)
   FROM tabl3) c;

Answer 3

我会在他们之间做CROSS JOIN：

SELECT * FROM
  (SELECT max(modifiedtime) FROM tabl1) a
    CROSS JOIN
  (SELECT max(modifiedtime) FROM tabl2) b
    CROSS JOIN
  (SELECT max(modifiedtime) FROM tabl3) c;