mysql-限制左联接

Question

左说，例如，我有以下查询，并想限制表的left join ，我该如何实现？

SELECT * FROM Base
LEFT JOIN Derived1 ON Base.id = Derived1.id
LEFT JOIN Derived2 ON Base.id = Derived2.id
LEFT JOIN Derived3 ON (Base.id = Derived3.id WHERE Derived3.name LIKE 'test%' LIMIT 5) <-----
WHERE Base.id = 'someid';

---

我虽然可以这样写：

SELECT * FROM Base
LEFT JOIN Derived1 ON Base.id = Derived1.id
LEFT JOIN Derived2 ON Base.id = Derived2.id
LEFT JOIN Derived3 ON Base.id = (
    SELECT d3.id FROM Derived3 d3
    WHERE d3.name LIKE 'test%' AND Base.id = Derived3.id
    LIMIT 5
)
WHERE Base.id = 'someid';

但这给了我错误的结果

我想所有的数据是内部Derived3 ，但只有5个项目，如果Derived.name等于test 1 ， test 2 ， test 3等，有可能是表内100个000 000测试和一些其他的东西千不启动与'test' 。

所以基本上，在Derived3内给我任何东西，最多5行以'test'开头。

我该如何解决？

Answer 1

检查波纹管查询。

SELECT *
FROM Base
LEFT JOIN Derived1 ON Base.id = Derived1.id
LEFT JOIN Derived2 ON Base.id = Derived2.id
LEFT JOIN Derived3 ON (
        Base.id = Derived3.id
        AND Derived3.id IN (
            SELECT Derived3.id
            FROM Derived3
            WHERE Derived3.NAME LIKE 'test%' LIMIT 5
            )
        )
WHERE Base.id = 'someid'

Answer 2

我不确定这是否适合您的情况，但是您可以使用过滤器LIKE 'test%'从Derived3中进行选择，并使用别名将选择结果加入。

我没有安装MySql来尝试此操作，但这样的方法应该可以工作：

SELECT * FROM Base
LEFT JOIN Derived1 ON Base.id = Derived1.id
LEFT JOIN Derived2 ON Base.id = Derived2.id
LEFT JOIN (
    SELECT d3.id FROM Derived3 d3
    WHERE d3.name LIKE 'test%'
    LIMIT 5
) Derived3Alias ON Base.id = Derived3Alias.id
WHERE Base.id = 'someid'
GROUP BY Base.id;

也许您需要编写“ as Derived3Alias”，但首先尝试不使用“ as”，我认为它应该可以工作。