【mysql】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

( 
    SELECT DISTINCT
        c.u_id,
        c.create_time

    FROM
        表A a
        LEFT JOIN 表B b ON b.u_id = a.u_id 
        AND b.yn = 1
        LEFT JOIN 表C c ON c.u_id = a.u_id 
        AND c.yn = 1
        LEFT JOIN 表D d ON d.u_id = a.u_id 
        AND d.yn = 1 
    WHERE
        1 = 1 
        AND a.yn = 1 
        AND b.vender_id IN ( 1 ) 
        AND b.join_behavior = 1 
        AND b.store_join_type = 1 
    ) UNION
    (
    SELECT DISTINCT
        c.u_id,
        c.create_time
        
    FROM
        表A a
        LEFT JOIN 表B b ON b.u_id = a.u_id
        LEFT JOIN 表C c ON c.u_id = a.u_id
        LEFT JOIN 表D d ON d.u_id = a.u_id 
    WHERE
        1 = 1 
        AND a.yn = 1 
        AND b.vender_id IN ( 1 ) 
        AND b.join_behavior = 1 
        AND a.join_value IN ( 6 ) 
        AND b.store_join_type = 2 
    ) 
ORDER BY
    create_time DESC 
    LIMIT 0,200;

count统计sql:

select 
     COUNT(*)
FROM
(
    ( SELECT DISTINCT
    c.u_id,
    c.create_time

    FROM
        表A a
        LEFT JOIN 表B b ON b.u_id = a.u_id 
        AND b.yn = 1
        LEFT JOIN 表C c ON c.u_id = a.u_id 
        AND c.yn = 1
        LEFT JOIN 表D d ON d.u_id = a.u_id 
        AND d.yn = 1 
    WHERE
        1 = 1 
        AND a.yn = 1 
        AND b.vender_id IN ( 1 ) 
        AND b.join_behavior = 1 
        AND b.store_join_type = 1 
    ) UNION
    (
    SELECT DISTINCT
        c.u_id,
        c.create_time
        
    FROM
        表A a
        LEFT JOIN 表B b ON b.u_id = a.u_id
        AND b.yn = 1
        LEFT JOIN 表C c ON c.u_id = a.u_id
        AND c.yn = 1
        LEFT JOIN 表D d ON d.u_id = a.u_id 
        AND d.yn = 1
    WHERE
        1 = 1 
        AND a.yn = 1 
        AND b.vender_id IN ( 1 ) 
        AND b.join_behavior = 1 
        AND a.join_value IN ( 6 ) 
        AND b.store_join_type = 2 
    ) 

)
AS temp