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