一文总结使用MySQL时遇到null值的坑

SELECT * FROM table_name WHERE column_name = NULL; -- 错误示例，永远返回空结果集

SELECT * FROM table_name WHERE column_name IS NULL; -- 正确示例，使用IS NULL判断

SELECT SUM(column_name) FROM table_name; -- 错误示例，忽略NULL值

SELECT SUM(COALESCE(column_name, 0)) FROM table_name; -- 正确示例，使用COALESCE函数处理NULL值

SELECT * FROM table1 JOIN table2 ON table1.column_name = table2.column_name; -- 错误示例，不包含NULL值的连接

SELECT * FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name; -- 正确示例，使用LEFT JOIN包含NULL值

INSERT INTO table_name (column_name) VALUES (NULL); -- 错误示例，插入NULL值

ALTER TABLE table_name ALTER COLUMN column_name SET DEFAULT default_value; -- 正确示例，设置列的默认值

CREATE TABLE table_name (
    column_name INT,
    foreign_key_column INT,
    FOREIGN KEY (foreign_key_column) REFERENCES other_table (column_name)
); -- 错误示例，未处理NULL值情况

CREATE TABLE table_name (
    column_name INT,
    foreign_key_column INT,
    FOREIGN KEY (foreign_key_column) REFERENCES other_table (column_name) ON DELETE SET NULL
); -- 正确示例，使用ON DELETE SET NULL处理NULL值