写了6、7年SQL，有写复杂，有写简单，但总体而言，基础是统一的，。就像编程之需要会加减乘除，用MySQL懂这些SQL就足够了。
目录
- 1. SELECT 查询
- 2. WHERE 子句
- 3. ORDER BY 子句
- 4. GROUP BY 子句
- 5. HAVING 子句
- 6. JOIN 查询
- 7. 子查询
- 8. INSERT INTO 查询
- 9. UPDATE 查询
- 10. DELETE 查询
- 11. LIMIT 子句
- 12. DISTINCT 关键字
MySQL 查询语句是用于从数据库中检索数据的语句。以下是一些常见的 MySQL 查询语句，并附详细说明和示例：
1. SELECT 查询
基本查询
SELECT column1, column2, ...
FROM table_name;
示例：
SELECT first_name, last_name
FROM employees;
从 employees 表中选择 first_name 和 last_name 字段。
查询所有列
SELECT *
FROM table_name;
示例：
SELECT *
FROM employees;
从 employees 表中选择所有列。
带条件的查询
SELECT column1, column2, ...
FROM table_name
WHERE condition;
示例：
SELECT first_name, last_name
FROM employees
WHERE department = 'Sales';
从 employees 表中选择 Sales 部门的 first_name 和 last_name 字段。
2. WHERE 子句
使用 AND 和 OR 逻辑运算符
SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND condition2 OR condition3;
示例：
SELECT first_name, last_name
FROM employees
WHERE department = 'Sales' AND salary > 50000;
从 employees 表中选择 Sales 部门且薪水大于 50000 的员工的 first_name 和 last_name 字段。
3. ORDER BY 子句
按指定列排序
SELECT column1, column2, ...
FROM table_name
ORDER BY column1 ASC|DESC, column2 ASC|DESC, ...;
示例：
SELECT first_name, last_name
FROM employees
ORDER BY last_name ASC, first_name DESC;
按 last_name 升序和 first_name 降序排列 employees 表中的数据。
4. GROUP BY 子句
按指定列分组并聚合数据
SELECT column1, COUNT(*)
FROM table_name
GROUP BY column1;
示例：
SELECT department, COUNT(*)
FROM employees
GROUP BY department;
按 department 列对 employees 表中的数据进行分组，并计算每个部门的员工数量。
5. HAVING 子句
对分组后的结果进行过滤
SELECT column1, COUNT(*)
FROM table_name
GROUP BY column1
HAVING condition;
示例：
SELECT department, COUNT(*)
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;
筛选出员工数量大于 5 的部门。
6. JOIN 查询
内连接
SELECT columns
FROM table1
INNER JOIN
table2
ON table1.column = table2.column;
示例：
SELECT employees.first_name, employees.last_name, departments.department_name
FROM employees
INNER JOIN departments
ON employees.department_id = departments.department_id;
将 employees 表和 departments 表进行内连接，选择 first_name、last_name 和 department_name。
左连接
SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column = table2.column;
示例：
SELECT employees.first_name, employees.last_name, departments.department_name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;
从 employees 表中选择所有记录，并匹配 departments 表中的记录，如果没有匹配到则返回 NULL。
右连接
SELECT columns
FROM table1
RIGHT JOIN table2
ON table1.column = table2.column;
示例：
SELECT employees.first_name, employees.last_name, departments.department_name
FROM employees
RIGHT JOIN departments
ON employees.department_id = departments.department_id;
从 departments 表中选择所有记录，并匹配 employees 表中的记录，如果没有匹配到则返回 NULL。
7. 子查询
在 SELECT 中使用子查询
SELECT column1
FROM table_name
WHERE column2 = (SELECT column
FROM another_table
WHERE condition);
示例：
SELECT first_name, last_name
FROM employees
WHERE department_id = (SELECT department_id
FROM departments
WHERE department_name = 'Sales');
从 employees 表中选择 Sales 部门的所有员工。
在 FROM 子句中使用子查询
SELECT subquery.column1, subquery.column2
FROM (SELECT column1, column2
FROM table_name
WHERE condition) AS subquery;
示例：
SELECT sub.first_name, sub.last_name
FROM (SELECT first_name, last_name
FROM employees
WHERE department_id = 1) AS sub;
从子查询结果中选择 department_id 为 1 的员工的 first_name 和 last_name。
8. INSERT INTO 查询
插入数据
INSERT INTO table_name (column1, column2, column3, ...)
VALUES (value1, value2, value3, ...);
示例：
INSERT INTO employees (first_name, last_name, department_id, salary)
VALUES ('John', 'Doe', 3, 50000);
向 employees 表中插入一条新记录。
9. UPDATE 查询
更新数据
UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;
示例：
UPDATE employees
SET salary = 55000
WHERE last_name = 'Doe';
更新 last_name 为 ‘Doe’ 的员工的 salary 为 55000。
10. DELETE 查询
删除数据
DELETE FROM table_name
WHERE condition;
示例：
DELETE FROM employees
WHERE last_name = 'Doe';
删除 last_name 为 ‘Doe’ 的员工记录。
11. LIMIT 子句
限制结果集
SELECT column1, column2, ...
FROM table_name
LIMIT number;
示例：
SELECT first_name, last_name
FROM employees
LIMIT 10;
选择前 10 条员工记录。
12. DISTINCT 关键字
去重查询
SELECT DISTINCT column1, column2, ...
FROM table_name;
示例：
SELECT DISTINCT department_id
FROM employees;
选择 employees 表中所有不同的 department_id。
这些示例涵盖了 MySQL 中的基本查询语句及其常见用法。通过这些示例，你可以构建更复杂的查询来满足不同的数据检索需求。
