在 mysql 中,多表查询是指在一个查询语句中同时使用多个表来获取所需的数据,这可以通过使用 join 子句来实现,比如常见的内连接、左连接、右连接、全连接、自连接等等多表查询方式! 下面我们不多废话,直接用一个简单的公司管理系统,有三张表 emp、dept、salgrade 来演示如何进行最简单的多表查询!(这...
( id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(50) NOT NULL, age INT CHECK (age >= 10), class_id INT, FOREIGN KEY (class_id) REFERENCES classes(id) ); -- 修改表结构 ALTER TABLE students ADD COLUMN email VARCHAR(100); ALTER TABLE students DROP COLUMN age; -- 删除表 DROP TABLE...
**from > on> join > where > group by > with > having > select(**含重命名) > distinct > order by > limit 三、Update | Delete | 插入查询结果 3.1 Update 语法: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 UPDATE table_name SET column = expr [, column = expr ...] [WHERE ...
交叉连接(CROSS JOIN)没有WHERE 子句,它返回连接表中所有数据行的笛卡尔积,其结果集合中的数据行数等于第一个表中符合查询条件的数据行数乘以第二个表中符合查询条件的数据行数。 连接操作中的ON (join_condition) 子句指出连接条件,它由被连接表中的列和比较运算符、逻辑运算符等构成。 无论哪种连接都不能对t...
LEFT JOIN departments d ON ed.department_id = d.id GROUP BY e.id """ cursor.execute(select_query) employees = cursor.fetchall() for emp in employees: print(f"ID: {emp['id']}, 姓名: {emp['first_name']} {emp['last_name']}, " ...
SET column_1 = value_1, column_2 = value_2, ... WHERE condition; 例如: 将选修“徐军”老师所教所有课堂的同学的成绩提高5% UPDATE sc JOIN teaching AS t ON sc.Cid = t.Cid JOIN teacher ON teacher.Tno = t.Tno JOIN student AS s ON sc.Sno = s.Sno ...
JOIN information_schema.constraint_column_usage ccu ON tc.constraint_name=ccu.constraint_name WHERE c.column_name='id'AND tc.constraint_type='PRIMARY KEY'AND c.column_default IS NULL AND c.data_type IN ('integer','bigint') AND c.table_schema='public'LOOP ...
UNION vs. JOIN# A JOIN combines result sets horizontally, a UNION appends result set vertically. The following picture illustrates the difference between UNION and JOIN: MySQL UNION and column alias examples# We will use the customers and employees tables in the sample database for the demonstrati...
Is it possible to perform a join on a certain table based on a value in the record itself? Suppose I've got three tables, POSTS, PERSONS_1 And PERSONS_2 Table PERSONS_1 contains personal data of persons of type 1 and Table PERSONS_2 contains personal data of persons of type 2 ...
左侧的表完全显示我们就说是左外连接;右侧的表完全显示我们就说是右外连接. 外连接也是多表查询的一种表现形式,一般使用比较少,属于特殊情况特殊处理.语法:-- 左外连接,表一完全显示select 字段名 from 表名1 left join 表名2 on 连接条件;-- 右外连接,表二完全显示select 字段名 from 表名1 right join ...