mysql - LEFT JOIN 两个表

Question

我有 3 个表，想根据他们的地区从“销售”中选择所有带有客户 ID 的记录。

create table t_sales (
   id                   SERIAL               not null,
   parent               INT4                 null,
   client1              TEXT                 null,
   client2              TEXT                 null,
   constraint PK_T_SALES primary key (id)
);
create table t_client (
   id                   SERIAL               not null,
   name                 TEXT                 null,
   district             INT4                 null,
   constraint PK_T_CLIENT primary key (id)
);
create table t_sale_district (
   id                   SERIAL               not null,
   sale_id              INT4                 null,
   district_id          INT4                 null,
   constraint PK_T_SALE_DISTRICT primary key (id)
);
INSERT INTO t_client (id,name,district) VALUES(1,'JOHN',1);
INSERT INTO t_client (id,name,district) VALUES(2,'JOHN',2);
INSERT INTO t_client (id,name,district) VALUES(3,'john',1);
INSERT INTO t_client (id,name,district) VALUES(4,'john',2);
INSERT INTO t_client (id,name,district) VALUES(5,'MAX',1);
INSERT INTO t_client (id,name,district) VALUES(6,'MAX',2);
INSERT INTO t_client (id,name,district) VALUES(7,'max',1);
INSERT INTO t_client (id,name,district) VALUES(8,'max',2);

INSERT INTO t_sales (id,parent,client1,client2) VALUES (1,1,'JOHN','john');
INSERT INTO t_sales (id,parent,client1,client2) VALUES (2,1,'JOHN','MAX');

INSERT INTO t_sale_district (id,sale_id,district_id) VALUES (1,1,1);
INSERT INTO t_sale_district (id,sale_id,district_id) VALUES(2,1,5);

INSERT INTO t_sales (id,parent,client1,client2) VALUES (3,1,'JOHN',NULL);
INSERT INTO t_sale_district (id,sale_id,district_id) VALUES (3,2,2);

这个查询选择了我想要的但没有空值:

SELECT s.id,s.client1 as c1,c1.district as d1,s.client2 as c2,c2.district as d2
FROM public.t_sales s
    JOIN t_client c1 ON c1.name=s.client1 
    JOIN t_client c2 ON c2.name=s.client2
    JOIN t_sale_district sd1 ON sd1.district_id = c1.district 
    JOIN t_sale_district sd2 ON sd2.district_id = c2.district 
    WHERE sd1.sale_id=s.id AND sd2.sale_id=s.id

1;"JOHN";1;"john";1
2;"JOHN";2;"MAX";2

但我想添加空值:

  1;"JOHN";1;"john";1
    2;"JOHN";2;"MAX";2
    3;"JOHN";2;NULL;NULL

是否可以使用销售中的空值进行此类查询？

PS 我知道 LEFT JOIN:

SELECT s.id,s.client1 as c1,c1.district as d1,s.client2 as c2,c2.district as d2
FROM public.t_sales s
LEFT JOIN t_client c1 ON c1.name=s.client1 
LEFT JOIN t_client c2 ON c2.name=s.client2
LEFT JOIN t_sale_district sd1 ON sd1.district_id = c1.district AND sd1.sale_id=s.id
LEFT JOIN t_sale_district sd2 ON sd2.district_id = c2.district AND sd2.sale_id=s.id

输出:

1;"JOHN";2;"john";1
1;"JOHN";1;"john";1
2;"JOHN";1;"MAX";2
2;"JOHN";2;"MAX";2
2;"JOHN";1;"MAX";1
2;"JOHN";2;"MAX";1
1;"JOHN";2;"john";2
1;"JOHN";1;"john";2
3;"JOHN";2;"";
3;"JOHN";1;"";

PS2。我只找到了一个解决方案(感谢 Zaynul Abadin Tuhin):

select ids,t1.* from    
(
    select s.id as ids,c1.district as d1, c2.district as d2,c1.id as c1,c2.id as c2,s.client1,s.client2 from  t_sales s    
     left join t_client c1 on s.client1=c1.name
     left join t_client c2 on s.client2=c2.name
 )
    as t1 
    left join    t_sale_district d1 on d1.sale_id=t1.ids 
    left join    t_sale_district d2 on d2.sale_id=t1.ids 
where
    (t1.d2 IS NULL OR t1.d2=d2.district_id )
    and (t1.d1 IS NULL OR t1.d1=d1.district_id)

输出:

1;1;1;1;1;3;"JOHN";"john"
2;2;2;2;2;6;"JOHN";"MAX"
3;3;1;;1;;"JOHN";""

有没有更简单的解决方案？

Answer 1

您可以尝试使用此查询，其中包含与您需要的结果完全相同的结果

SELECT s.id,s.client1 as c1,c1.district as d1,s.client2 as c2,c2.district as d2
FROM public.t_sales s
LEFT JOIN t_client c1 ON c1.name=s.client1 
LEFT JOIN t_client c2 ON c2.name=s.client2
LEFT JOIN t_sale_district sd1 ON sd1.district_id = c1.district AND sd1.sale_id=s.id
LEFT JOIN t_sale_district sd2 ON sd2.district_id = c2.district AND sd2.sale_id=s.id