常用SQL命令大全

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：

1. SELECT - 查询数据

SELECT column1, column2, ...
FROM table_name
WHERE condition;

示例：

SELECT name, age
FROM students
WHERE age > 20;

2. INSERT - 插入数据

INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);

示例：

INSERT INTO students (name, age)
VALUES ('Alice', 25);

3. 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';

4. DELETE - 删除数据

DELETE FROM table_name
WHERE condition;

示例：

DELETE FROM students
WHERE age < 18;

5. 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
);

6. ALTER TABLE - 修改表结构

ALTER TABLE table_name
ADD column_name datatype;

示例：

ALTER TABLE students
ADD email VARCHAR(100);

7. DROP TABLE - 删除表

DROP TABLE table_name;

示例：

DROP TABLE students;

8. 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;

9. 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;

10. 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;

11. 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;

12. 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。