SQL空值判断，双表关联数据统计查询实例讲解

SQL空值判断

oracle的是NVL(value,nullvalue)

pgsql的是COALESCE(value,nullvalue)

mysql的是IFNULL(value,nullvalue)

双表关联数据统计查询

一个表是数据，一个表是项目，关联数据和项目表，查看每个项目有多少人已经有检查结果。

 SELECT result2.facility_type_num,
    result2.facility_type_name,
    COALESCE(result1.countnum, (0)::bigint) AS "coalesce"
   FROM (( SELECT project.facility_type_num,
            project.facility_type_name,
            0 AS countnum
           FROM check_project project) result2
     LEFT JOIN ( SELECT cp.facility_type_num,
            cp.facility_type_name,
            sum(1) AS countnum
           FROM (check_project cp
             JOIN check_result r ON ((((r.facility_type_num)::text = (cp.facility_type_num)::text) AND (to_char(r.lastuptime, 'yyyy-MM-dd'::text) = to_char((CURRENT_DATE)::timestamp with time zone, 'yyyy-MM-dd'::text)))))
          GROUP BY cp.facility_type_num, cp.facility_type_name
          ORDER BY cp.facility_type_num) result1 ON (((result1.facility_type_num)::text = (result2.facility_type_num)::text)))