Question

简单地说：

SELECT
  table1.field1,
  table2.field2,
  table3.field_WHAT
FROM
  table1,
  table2,
  table3
WHERE
  table1.field1 = 'HELP'
  and table2.field1 = table1.field1
  and table3.field2 = table2.field2

我仍然想要table1＆amp;的所有匹配行table2，即使找不到table3.field2的匹配记录...而理想情况如果没有匹配，table3.field_WHAT将返回""或NULL。

Answer 1

试试这个：

SELECT
  table1.field1,
  table2.field2,
  table3.field_WHAT
FROM
  table1
INNER JOIN
  table2 ON table1.field1 = table2.field1
LEFT OUTER JOIN
  table3 ON table3.field2 = table2.field2
WHERE
  table1.field1 = 'HELP'

Answer 2

这是outer join的经典案例：

SELECT
  table1.field1,
  table2.field2,
  table3.field_WHAT
FROM
  table1,
JOIN 
  table2 ON table2.field1 = table1.field1
OUTER JOIN
  table3 ON table3.field2 = table2.field2
WHERE
  table1.field1 = 'HELP'

当一个表返回0时，mysql返回匹配其他行

2 个答案: