mysql如何查询字段值为null或not null的数据

查询Topic表中Category值为null的数据

SELECT Name, Department, Category
FROM Topic
WHERE Category IS NULL
ORDER BY Name;

或者

SELECT Name, Department, Category
FROM Topic
WHERE Category <=>NULL
ORDER BY Name;

查询Topic表中Category值不为null的数据（ http://www.manongjc.com/article/1408.html ）

SELECT Name, Department, Category
FROM Topic
WHERE Category IS NOT NULL
ORDER BY Name;