多步骤查询的解决方案

使用SQL VIEW和CTE(common table expression)去把复杂查询简化，而不要使用JOIN，影响性能。

WITH step1 AS

 (SELECT 1,2 

  FROM orders o, src1 c

  WHERE c.key1 = o.key1 AND con1=1),

step2 AS

(SELECT 3,4

  FROM step1, src2 s

  WHERE step1.key2 = s.key2 AND con2='2'),

step3 AS

(SELECT 5,6

  FROM step2, src3

  WHERE x = y)

SELECT * FROM step3