sql将两张表合并为一张，并按指定条件将多行记录合并为一行

需求：

       按照项目ID，日期 将以下两张表数据合并

  表1                                                                                   表2                                 

                       

 

目标表：

 

解决思路：

1. 将不同的表转化为统一结构的表， 不存在的字段默认设置为NULL；  # NULL AS total_cases
2. 去重合并两张表格 UNION；
3. 将字段值为null的设置为0  ， 按指定字段分组并求和；  # IFNULL(num, 0)  

SQL语句：

SELECT SUM(t4.interface_num) AS interface_num, SUM(t4.total_cases) AS total_cases, t4.date, t4.project_id FROM(
SELECT IFNULL(t3.interface_num,0) AS interface_num, IFNULL(t3.total_cases,0) total_cases, date, t3.project_id FROM
(SELECT t1.interface_num, NULL AS total_cases, t1.interface_date AS date, t1.project_id FROM tb1_name t1
UNION
SELECT NULL AS interface_num, t2.total_cases, t2.case_date AS date, t2.project_id FROM tb2_name t2) t3 

) t4
GROUP BY t2.date, t2.project_id
ORDER BY date;