exists 和 not exists 的用法（括号里 select 1 和 select count（1） 是会产生不同的结果的）

1.

#  exists 和 not  exists 的区别：
#  ① exists : exists 后面括号里返回的是 "true" ，则 当前记录满足条件 (包含：查总数发现 count 是 1 ，true 满足要求)
#     not  exists :  not  exists  返回的是 "false" ,则 当前记录满足条件(不包含：查总数发现 count 是 0 满足要求)
#  ②  用法：
#          exists （select  1  from B）
#          not  exists （select  1  from B）
#          错误用法：
#              exists （select  count(1)  from B）
#              not  exists （select  count(1)  from B）
#     说明：  select 1  这里的  1  其实是 无关紧要的，换成 * , 0 也没问题，他只在乎括号里的数据能不能查得出来,是否存在这样的记录，如果存在 ，当前数据就满足条件
#      能查出来的情况：
         # +-+
         # |1|
         # +-+
         # |1|
         # +-+
#      不能查出来的情况
#       # +-+
#       # |1|
#       # +-+
select * from boy ;

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# |4 |慕容复 |
# +--+----+

select * from girl  ;

# +--+----+------+
# |id|name|boy_id|
# +--+----+------+
# |1 |A   |1     |
# |2 |B   |2     |
# |3 |C   |3     |
# +--+----+------+


# 有女朋友的 boy （exists ）
select * from boy where exists(select  1  from girl where boy.id =  girl.boy_id);

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# +--+----+


#  将  1  换成 *

select * from boy where exists(select  *  from girl where boy.id =  girl.boy_id);

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# +--+----+


#  错误写法: 原因：boy 为 4 ： select  count(1)  from girl where   girl.boy_id = 4  查到的结果 是  0 , 0 也是 值
select  count(1)  from girl where   girl.boy_id = 4 ;
# +--------+
# |count(1)|
# +--------+
# |0       |
# +--------+


#  没值的情况
select  1 as  A from girl where   girl.boy_id = 4 ;
# +-+
# |1|
# +-+

#  没值的情况
select  *  from girl where   girl.boy_id = 4 ;
# +-+
# |1|
# +-+

#  有值的情况(满足条件的情况)

select  1  from girl where   girl.boy_id = 3 ;

# +-+
# |1|
# +-+
# |1|
# +-+

select  0  from girl where   girl.boy_id = 3 ;

# +-+
# |0|
# +-+
# |0|
# +-+

select  *  from girl where   girl.boy_id = 3 ;
# +--+----+------+
# |id|name|boy_id|
# +--+----+------+
# |3 |C   |3     |
# +--+----+------+

select * from boy where exists(select  count(1)  from girl where boy.id =  girl.boy_id);

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# |4 |慕容复 |
# +--+----+

#
select * from boy where exists(select  count(*)  from girl where boy.id =  girl.boy_id);

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# |4 |慕容复 |
# +--+----+


# 不推荐写法：
select * from boy where exists(select  count(1)  from girl where boy.id =  girl.boy_id  group by  girl.boy_id having count(1)  >=1  );

# +--+----+
# |id|name|
# +--+----+
# |1 |慕容皝 |
# |2 |慕容垂 |
# |3 |慕容博 |
# +--+----+


#  not  exists  的用法： 单身男生


select * from boy where not exists(select  1  from girl where boy.id =  girl.boy_id );

# +--+----+
# |id|name|
# +--+----+
# |4 |慕容复 |
# +--+----+

#  not exists 不存在 （） ，括号里 查出来没有，就代表不存在

select  1  from girl where   girl.boy_id =  4  ;

# 没有
# +-+
# |1|
# +-+


select  1  from girl where   girl.boy_id in(1,2,3)     ;

# +-+
# |1|
# +-+
# |1|
# |1|
# |1|
# +-+