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SQL（Structured Query Language）是一种用于存储、操作和检索数据库中数据的标准编程语言。以下是一些常用的 SQL 语法：

1. 数据库操作

   • 创建数据库：CREATE DATABASE database_name;
   • 删除数据库：DROP DATABASE database_name;
   • 选择数据库：USE database_name;

2. 表操作

   • 创建表：CREATE TABLE table_name (column1 datatype, column2 datatype, ...);
   • 删除表：DROP TABLE table_name;
   • 修改表结构：ALTER TABLE table_name ADD column_name datatype;
   • 删除表中的列：ALTER TABLE table_name DROP COLUMN column_name;

3. 数据操作

   • 插入数据：INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
   • 更新数据：UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
   • 删除数据：DELETE FROM table_name WHERE condition;
   • 查询数据：SELECT column1, column2, ... FROM table_name WHERE condition;

4. 条件查询

   • 等于：SELECT * FROM table_name WHERE column = value;
   • 不等于：SELECT * FROM table_name WHERE column != value;
   • 大于：SELECT * FROM table_name WHERE column > value;
   • 大于等于：SELECT * FROM table_name WHERE column >= value;
   • 小于：SELECT * FROM table_name WHERE column < value;
   • 小于等于：SELECT * FROM table_name WHERE column <= value;
   • BETWEEN：SELECT * FROM table_name WHERE column BETWEEN value1 AND value2;
   • LIKE（模糊查询）：SELECT * FROM table_name WHERE column LIKE 'value%';
   • IN：SELECT * FROM table_name WHERE column IN (value1, value2, ...);
   • AND/OR：SELECT * FROM table_name WHERE column1 = value1 AND column2 = value2;

5. 聚合函数

   • COUNT：SELECT COUNT(*) FROM table_name;
   • SUM：SELECT SUM(column) FROM table_name;
   • AVG：SELECT AVG(column) FROM table_name;
   • MAX：SELECT MAX(column) FROM table_name;
   • MIN：SELECT MIN(column) FROM table_name;

6. 分组和排序

   • 分组：SELECT column1, COUNT(*) FROM table_name GROUP BY column1;
   • 排序：SELECT * FROM table_name ORDER BY column1 ASC, column2 DESC;

7. 连接查询

   • 内连接（INNER JOIN）：SELECT * FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name;
   • 左连接（LEFT JOIN）：SELECT * FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;
   • 右连接（RIGHT JOIN）：SELECT * FROM table1 RIGHT JOIN table2 ON table1.column_name = table2.column_name;
   • 全连接（FULL JOIN）（注意：并非所有数据库系统都支持 FULL JOIN）：SELECT * FROM table1 FULL JOIN table2 ON table1.column_name = table2.column_name;

8. 子查询

   • 嵌套查询：SELECT * FROM table_name WHERE column IN (SELECT column FROM other_table WHERE condition);

9. 事务控制

   • 开启事务：BEGIN TRANSACTION;
   • 提交事务：COMMIT;
   • 回滚事务：ROLLBACK;

10. 权限控制

    • 授予权限：GRANT SELECT, INSERT ON table_name TO user_name;
    • 撤销权限：REVOKE SELECT, INSERT ON table_name FROM user_name;