SQL(Structured Query Language)是一种用于管理关系数据库管理系统的标准编程语言。它用于执行各种操作,如查询、更新、插入和删除数据库中的数据。以下是一些常用的SQL命令:
- SELECT - 查询数据
| SELECT column1, column2, ... |
| FROM table_name |
| WHERE condition; |
示例:
| SELECT name, age |
| FROM students |
| WHERE age > 20; |
- INSERT - 插入数据
| INSERT INTO table_name (column1, column2, ...) |
| VALUES (value1, value2, ...); |
示例:
| INSERT INTO students (name, age) |
| VALUES ('Alice', 25); |
- UPDATE - 更新数据
| UPDATE table_name |
| SET column1 = value1, column2 = value2, ... |
| WHERE condition; |
示例:
| UPDATE students |
| SET age = 26 |
| WHERE name = 'Alice'; |
- DELETE - 删除数据
| DELETE FROM table_name |
| WHERE condition; |
示例:
| DELETE FROM students |
| WHERE age < 18; |
- CREATE TABLE - 创建新表
| CREATE TABLE table_name ( |
| column1 datatype, |
| column2 datatype, |
| ... |
| ); |
示例:
| CREATE TABLE students ( |
| id INT PRIMARY KEY, |
| name VARCHAR(50), |
| age INT |
| ); |
- ALTER TABLE - 修改表结构
| ALTER TABLE table_name |
| ADD column_name datatype; |
示例:
| ALTER TABLE students |
| ADD email VARCHAR(100); |
- DROP TABLE - 删除表
示例:
- JOIN - 表连接
| SELECT column_name(s) |
| FROM table1 |
| JOIN table2 |
| ON table1.column_name = table2.column_name; |
示例:
| SELECT students.name, courses.course_name |
| FROM students |
| JOIN courses |
| ON students.course_id = courses.id; |
- ORDER BY - 排序
| SELECT column_name(s) |
| FROM table_name |
| ORDER BY column_name ASC|DESC; |
示例:
| SELECT name, age |
| FROM students |
| ORDER BY age DESC; |
- GROUP BY - 分组
| SELECT column_name(s), AGGREGATE_FUNCTION(column_name) |
| FROM table_name |
| WHERE condition |
| GROUP BY column_name; |
示例:
| SELECT age, COUNT(name) as total_students |
| FROM students |
| GROUP BY age; |
- HAVING - 分组后的条件过滤
| SELECT column_name(s), AGGREGATE_FUNCTION(column_name) |
| FROM table_name |
| WHERE condition |
| GROUP BY column_name |
| HAVING condition; |
示例:
| SELECT age, COUNT(name) as total_students |
| FROM students |
| GROUP BY age |
| HAVING COUNT(name) > 10; |
- UNION - 合并查询结果
| SELECT column_name(s) FROM table1 |
| UNION |
| SELECT column_name(s) FROM table2; |
示例:
| SELECT name FROM students |
| UNION |
| SELECT name FROM teachers; |
这些是SQL中的一些基本命令,每个命令都有其特定的用途和语法。通过组合这些命令,你可以进行复杂的数据库操作。
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