Question

我想做这样的事情：

SELECT
    Users.id,
    Countries.name 
FROM
    Users, Countries
WHERE 
    Users.country_id = Countries.id

问题是Users.country_id有时可能是NULL，如果是，则不返回整行。我希望它返回行但在该列中使用'null'或''。我试过这个：

SELECT
    Users.id,
    Countries.name 
FROM
    Users, Countries
WHERE 
    (Users.country_id = Countries.id OR Users.country_id IS NULL)

然而，对于存在的每个 * Countries.id * ， 会返回一行 > 即可。我该如何解决这个问题？

Answer 1

SELECT  users.id, countries.name
FROM    users
LEFT JOIN
        countries
ON      countries.id = users.country_id

这是一个很好的示例，说明为什么ANSI加入语法（使用JOIN关键字）是首选。

Answer 2

SELECT          Users.id,
                Countries.name
FROM            Users
LEFT OUTER JOIN Countries ON Users.country_id = Countries.id

Answer 3

SELECT
    Users.id,
    Countries.name 
FROM
    Users, Countries
WHERE 
    Users.country_id = Countries.id
UNION
SELECT
    id,
    '{{no country}}' AS name 
FROM
    Users
WHERE 
   NOT EXISTS ( SELECT * 
                  FROM Countries
                 WHERE Users.country_id = Countries.id );

在可能包含NULL的列上加入

3 个答案: