Question

我正在尝试执行一个简单的mysql连接：

我有matches的表格，其中包含两个我要加入的字段：team1和team2。

我想找到存储在teams表格中的团队的相应名称：

SELECT teams.team_name AS "name1", teams.team_name AS "name2", matches.id
FROM teams, matches 
WHERE matches.id=1
AND matches.team1_id=teams.team_id
AND matches.team2_id=teams.team_id

如果我删除了最后一个和条件我得到了一个结果，但如果我同时包括两个我得到一个空集？

我做错了什么？

Answer 1

对于要在查询中唯一引用的每个团队，您需要一个单独的别名，否则SQL将比较同一行。

SELECT team1.team_name AS "name1", team2.team_name AS "name2", matches.id
FROM teams team1, teams team2, matches 
WHERE matches.id=1
AND matches.team1_id=team1.team_id
AND matches.team2_id=team2.team_id

现在我们为team1和team2提供了2个team表的别名，因此他们可以分别引用不同的行。

Answer 2

您需要在表teams上加入matches两次，因为其中两列依赖于它，

SELECT  a.*,                -- <<== select column that you want to project
        b.team_name AS Team1Name,
        c.Team_name AS Team2Name
FROM    matches a
        INNER JOIN teams b
            ON a.team1_ID = b.team_ID
        INNER JOIN teams c
            ON a.team2_ID = c.team_ID
-- WHERE a.id = 1

要进一步了解联接，请访问以下链接：

Visual Representation of SQL Joins

Answer 3

您的查询将只返回团队自身对抗的记录（当然不会发生）。你想要两个不同的团队联盟，所以它有两个实例：

select
    teams1.team_name as "name1"
    ,teams2.team_name as "name2"
from
    matches
join
    teams teams1 on matches.team1_id = teams1.team_id
join
    teams teams2 on matches.team2_id = teams2.team_id
where
    matches.id = 1

Answer 4

试试这个：

SELECT team1.team_name AS "name1", team2.team_name AS "name2", matches.id
FROM teams as team1, teams as team2, matches 
WHERE matches.id=1
AND matches.team1_id=team1.team_id
AND matches.team2_id=team2.team_id

mysql简单连接

4 个答案: