mysql union查询，实现分页查询和count统计

说明：

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。

请注意，UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同。否则会报错。


union和 union all的关系和区别：
    1.UNION ALL 命令和 UNION 命令几乎是等效的，不过 UNION ALL 命令会列出所有的值。
    
    2.即 ：union会对合并的两个或多个查询的结果集 进行 去重合并的操作。
            union all则不会去重，只做合并，因此查出的结果集中 会有重复的记录

示例：

分页查询sql

 1 ( 
 2     SELECT DISTINCT
 3         c.u_id,
 4         c.create_time
 5  
 6     FROM
 7         表A a
 8         LEFT JOIN 表B b ON b.u_id = a.u_id 
 9         AND b.yn = 1
10         LEFT JOIN 表C c ON c.u_id = a.u_id 
11         AND c.yn = 1
12         LEFT JOIN 表D d ON d.u_id = a.u_id 
13         AND d.yn = 1 
14     WHERE
15         1 = 1 
16         AND a.yn = 1 
17         AND b.vender_id IN ( 1 ) 
18         AND b.join_behavior = 1 
19         AND b.store_join_type = 1 
20     ) UNION
21     (
22     SELECT DISTINCT
23         c.u_id,
24         c.create_time
25         
26     FROM
27         表A a
28         LEFT JOIN 表B b ON b.u_id = a.u_id
29         LEFT JOIN 表C c ON c.u_id = a.u_id
30         LEFT JOIN 表D d ON d.u_id = a.u_id 
31     WHERE
32         1 = 1 
33         AND a.yn = 1 
34         AND b.vender_id IN ( 1 ) 
35         AND b.join_behavior = 1 
36         AND a.join_value IN ( 6 ) 
37         AND b.store_join_type = 2 
38     ) 
39 ORDER BY
40     create_time DESC 
41     LIMIT 0,200;

count统计sql:

 1 select 
 2      COUNT(*)
 3 FROM
 4 (
 5     ( SELECT DISTINCT
 6     c.u_id,
 7     c.create_time
 8  
 9     FROM
10         表A a
11         LEFT JOIN 表B b ON b.u_id = a.u_id 
12         AND b.yn = 1
13         LEFT JOIN 表C c ON c.u_id = a.u_id 
14         AND c.yn = 1
15         LEFT JOIN 表D d ON d.u_id = a.u_id 
16         AND d.yn = 1 
17     WHERE
18         1 = 1 
19         AND a.yn = 1 
20         AND b.vender_id IN ( 1 ) 
21         AND b.join_behavior = 1 
22         AND b.store_join_type = 1 
23     ) UNION
24     (
25     SELECT DISTINCT
26         c.u_id,
27         c.create_time
28         
29     FROM
30         表A a
31         LEFT JOIN 表B b ON b.u_id = a.u_id
32         AND b.yn = 1
33         LEFT JOIN 表C c ON c.u_id = a.u_id
34         AND c.yn = 1
35         LEFT JOIN 表D d ON d.u_id = a.u_id 
36         AND d.yn = 1
37     WHERE
38         1 = 1 
39         AND a.yn = 1 
40         AND b.vender_id IN ( 1 ) 
41         AND b.join_behavior = 1 
42         AND a.join_value IN ( 6 ) 
43         AND b.store_join_type = 2 
44     ) 
45  
46 )
47 AS temp