Mysql exists用法小结

select * from student; 
select * from student where exists (select 1); 
select * from student where exists (select null);

select * from student a 
where exists (select sno from student_course_relation b where b.cno=3 and b.sno=a.sno); 
select * from student a 
where exists (select cno from student_course_relation b where b.cno=3 and b.sno=a.sno); 
select * from student a 
where exists (select 1 from student_course_relation b where b.cno=3 and b.sno=a.sno);

CREATE TABLE `activity_main` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`act_code` varchar(255) NOT NULL COMMENT '活动代码',
`act_name` varchar(255) NOT NULL COMMENT '活动名称',
PRIMARY KEY (`id`),
UNIQUE KEY `uniq_code` (`act_code`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT='活动主表'

CREATE TABLE `activity_area` (
 `id` bigint(20) NOT NULL AUTO_INCREMENT,
 `act_code` varchar(255) NOT NULL COMMENT '活动代码',
 `area` varchar(255) NOT NULL COMMENT '参与此活动的网站',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT='活动适配的网站列表'

CREATE TABLE `activity_sku` (
 `id` bigint(20) NOT NULL AUTO_INCREMENT,
 `act_code` varchar(255) NOT NULL COMMENT '活动代码',
 `sku` varchar(255) NOT NULL COMMENT '活动赠送的商品',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT='活动赠品表'

# 查询体重秤
select * from activity_main where act_code in (
select act_code from activity_sku where sku = '翎野君的体脂称'
)

# 查询体重秤
select * from activity_main a where exists (
select 1 from activity_sku b where a.act_code = b.act_code and b.sku = '翎野君的体脂称'
)

# 模糊查询B-BEKO英国婴儿推车
select * from activity_main where act_code in (
select act_code from activity_sku where sku like '%B-BEKO%'
)

# 模糊查询B-BEKO英国婴儿推车
select * from activity_main a where exists (
select 1 from activity_sku b where a.act_code = b.act_code and b.sku like '%B-BEKO%'
)

# 查询在博客园举办的活动
select * from activity_main where act_code in (
select act_code from activity_area where area = '博客园'
)

# 查询在博客园举办的活动
select * from activity_main a where exists (
select 1 from activity_area b where a.act_code = b.act_code and b.area = '博客园'
)


# 在博客园举办活动且活动奖品为华为手机的活动信息
select * from activity_main where act_code in (
select act_code from activity_area where area = '博客园' and act_code in (
select act_code from activity_sku where sku = '华为P30Pro'
))


# 内层的exists语句只在当前where语句中生效，最终是否返回，要根据最外层的exists判断，如果是 true（真）就返回到结果集，为 false（假）丢弃。
select * from activity_main a where exists (
select 1 from activity_area b where a.act_code = b.act_code and b.area = '博客园' and exists
(select 1 from activity_sku c where a.act_code = c.act_code and c.sku = '华为P30Pro')
)