同理的查询语句，嵌套能查出来，直接查不出来。

Question

求解！
下面两个语句，为什么第一个很快，第二个查不出来，是表锁住了么？
注：seq_next_table就是一个返回序列的函数。

SELECT md5(rand() +seq_next_table('cc')), dist_no,id 
FROM (SELECT  b.dist_no,count(distinct a.id) id
  from t_customer a,cxj_o_dist_region_rela b 
  where  SUBSTR( a.region_code, 3, 2 )=b.region  group by b.dist_no  )asdas

SELECT md5(rand() +seq_next_table('cc')), b.dist_no,count(distinct a.id)
FROM t_customer a,cxj_o_dist_region_rela b 
WHERE  SUBSTR( a.region_code, 3, 2 )=b.region  group by b.dist_no 

Answer 1

用join来连接看看

SELECT md5(rand() +seq_next_table('cc')), b.dist_no,count(distinct a.id)
FROM t_customer a
JOIN cxj_o_dist_region_rela b ON SUBSTR(a.region_code, 3, 2) = b.region
GROUP BY b.dist_no;