左联接显示联接 table 中的多个值

Question

有这个SQL:

create table Team(id integer, attacker int, midfield int, defender int);
insert into Team(id, attacker, midfield, defender) values(1, 50, 51, 51);
insert into Team(id, attacker, midfield, defender) values(2, 50, NULL, 55);
insert into Team(id, attacker, midfield, defender) values(12, 50, 53, NULL);


create table Footballer(id integer, name varchar(100));
insert into Footballer(id, name) values (50, "AbbaDude");
insert into Footballer(id, name) values (51, "BravoDude");
insert into Footballer(id, name) values (52, "Charlie");
insert into Footballer(id, name) values (53, "Dude");
insert into Footballer(id, name) values (54, "Elfonso");
insert into Footballer(id, name) values (55, "Fix");

select t.id, t.attacker, t.midfield, t.defender, f.name from Team t 
left join Footballer f on f.id = t.id

我明白了：

id  attacker    midfield    defender    name
1   50          51          51          NULL
2   50          NULL        55          NULL
12  50          53          NULL        NULL

我想查看来自 table Footballer.

的名字，而不是数字（50、51、55 等）

id  attacker    midfield    defender    
1   AbbaDude    BravoDude   BravoDude   
2   AbbaDude    NULL        Fix         
12  AbbaDude    Dude        NULL

Answer 1

您需要 Footballer table 中的时间值。所以，您必须使用此代码：

create table Footballer(id integer,teamId integer , name varchar(100))
insert into Footballer(id, name) values (50,[teamId], "AbbaDude");
insert into Footballer(id, name) values (51,,[teamId], "BravoDude");

select t.id, t.attacker, t.midfield, t.defender, f.name from Team t 
left join Footballer f on f.teamId = t.id

所有代码：

create table Team(id integer, attacker int, midfield int, defender int);
insert into Team(id, attacker, midfield, defender) values(1, 50, 51, 51);
insert into Team(id, attacker, midfield, defender) values(2, 50, NULL, 55);
insert into Team(id, attacker, midfield, defender) values(12, 50, 53, NULL);


create table Footballer(id integer,teamID INTEGER, name varchar(100));
insert into Footballer(id,teamID, name) values (50,1, 'AbbaDude');
insert into Footballer(id,teamID, name) values (51,2, 'BravoDude');
insert into Footballer(id,teamID, name) values (52,1, 'Charlie');
insert into Footballer(id,teamID, name) values (53,12, 'Dude');
insert into Footballer(id,teamID, name) values (54,12, 'Elfonso');
insert into Footballer(id,teamID, name) values (55,1, 'Fix');

select t.id, t.attacker, t.midfield, t.defender, f.name from Team t 
left join Footballer f on f.teamID = t.id

Answer 2

您可以使用此查询：

select t.id, at.name, md.name, df.name 
from Team t 
left join Footballer at on at.id = t.attacker
left join Footballer md on md.id = t.midfield
left join Footballer df on df.id = t.defender

Answer 3

您需要加入 table Footballer 三次，因此我们使用别名来了解我们指的是哪个加入。我喜欢用一个或两个字母的别名，但是一个字母让你知道你指的是什么。
这里a代表前锋，m代表中场，d代表后卫。

select
  t.id,
  a.name attacker,
  m.name midfield,
  d.name defender
from Team t
left join Footballer a on t.attacker = a.id
left join Footballer m on t.midfield = m.id
left join Footballer d on t.defender = d.id

id | attacker | midfield  | defender 
-: | :------- | :-------- | :--------
 1 | AbbaDude | BravoDude | BravoDude
 2 | AbbaDude | null      | Fix      
12 | AbbaDude | Dude      | null

db<>fiddle here

左联接显示联接 table 中的多个值

Left Join showing multiple values from the joined table

mysql

sql

join

left-join