【MySQL成神之路】MySQL查询用法总结

MySQL查询语句全面指南

一、基础查询语句

MySQL中select的基本语法形式

select 属性列表
from 表名和视图列表
[where 条件表达式]
[group by 属性名[having 条件表达式]]
[order by 属性名[asc|desc]]
[limit <offset>,row count]

说明：

where子句：按照"条件表达式"指定的条件进行查询。

group by子句：按照"属性名"指定的字段进行分组。

having子句：有group by才能having子句，只有满足"条件表达式"中指定的条件的才能够输出。group by子句通常和count()、sum()等聚合函数一起使用。

order by子句：按照"属性名"指定的字段进行排序。排序方式由"asc"和"desc"两个参数指出，默认是按照"asc"来排序，即升序。

1. SELECT基本查询

 查询所有列 SELECT * FROM employees; 
 查询特定列 SELECT first_name, last_name, salary FROM employees; 
 使用DISTINCT去重 SELECT DISTINCT department_id FROM employees; 

2. WHERE条件查询

基本条件 SELECT * FROM employees WHERE salary > 5000; 
多条件组合 SELECT * FROM employees WHERE department_id = 10 AND salary > 6000; 
使用BETWEEN范围查询 SELECT * FROM employees WHERE salary BETWEEN 4000 AND 8000; 

二、高级查询功能

1. 聚合函数与GROUP BY

 常用聚合函数 SELECT COUNT(*) AS total_employees,
 AVG(salary) AS avg_salary,
 MAX(salary) AS max_salary,
 MIN(salary) AS min_salary, 
SUM(salary) AS total_salary FROM employees; 

 GROUP BY分组 SELECT department_id, COUNT(*) AS emp_count,
 AVG(salary) FROM employees GROUP BY department_id; 

2. HAVING子句

 筛选分组结果 SELECT department_id,
 AVG(salary) AS avg_salary FROM employees
 GROUP BY department_id HAVING AVG(salary) > 5000; 

3. 多表连接查询

内连接 SELECT e.first_name, e.last_name, d.department_name 
FROM employees e INNER JOIN departments d ON e.department_id = d.department_id; 

左外连接 SELECT e.first_name, e.last_name, d.department_name 
FROM employees e LEFT JOIN departments d ON e.department_id = d.department_id;

自连接 SELECT e1.employee_id, e1.first_name, e2.first_name AS manager_name 
FROM employees e1 LEFT JOIN employees e2 ON e1.manager_id = e2.employee_id; 

三、子查询与复杂查询

1. 子查询

 WHERE子句中的子查询 SELECT first_name, last_name, salary 
FROM employees WHERE salary > (SELECT AVG(salary) FROM employees); 

 FROM子句中的子查询 SELECT dept_avg.department_id, dept_avg.avg_salary 
FROM ( SELECT department_id, AVG(salary) AS avg_salary FROM employees
GROUP BY department_id ) AS dept_avg WHERE dept_avg.avg_salary > 6000; 

2. EXISTS/NOT EXISTS

查找有下属的管理者 
SELECT first_name, last_name 
FROM employees e WHERE EXISTS ( SELECT 1 FROM employees 
WHERE manager_id = e.employee_id ); 

3. 公用表表达式(CTE)

-- 使用WITH子句创建
CTE WITH dept_stats AS
 ( SELECT department_id, 
COUNT(*) AS emp_count, 
AVG(salary) AS avg_salary FROM employees 
GROUP BY department_id ) 
SELECT * FROM dept_stats 
WHERE emp_count > 5 ORDER BY avg_salary DESC; 

四、查询优化技巧

1. 索引使用最佳实践

-- 创建合适索引 
CREATE INDEX idx_emp_dept ON employees(department_id); 
CREATE INDEX idx_emp_name ON employees(last_name, first_name);
-- 强制使用索引 
SELECT * FROM employees FORCE INDEX(idx_emp_dept) WHERE department_id = 10; 

2. EXPLAIN分析查询

-- 分析查询执行计划 
EXPLAIN SELECT e.first_name, d.department_name 
FROM employees e JOIN departments d ON e.department_id = d.department_id 
WHERE e.salary > 5000; 

五、实用查询技巧

1. 条件逻辑

-- CASE表达式 
SELECT first_name, last_name, salary, CASE 
WHEN salary < 5000 THEN '初级' 
WHEN salary BETWEEN 5000 AND 10000 THEN '中级' ELSE '高级' END AS level FROM employees; 

2. 窗口函数

-- 排名函数 
SELECT first_name, last_name, salary, 
RANK() OVER (ORDER BY salary DESC) AS salary_rank, 
DENSE_RANK() OVER (ORDER BY salary DESC) AS salary_dense_rank,
ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_num FROM employees; 
-- 分区窗口函数 
SELECT department_id, first_name, last_name, salary, 
AVG(salary) OVER (PARTITION BY department_id) AS dept_avg_salary FROM employees; 

3. JSON处理

-- JSON数据查询 
SELECT employee_id, JSON_EXTRACT(profile, '$.skills') AS skills, 
JSON_CONTAINS(profile, '"MySQL"', '$.skills') AS has_mysql FROM employees_json; 
-- 更新JSON字段 
UPDATE employees_json SET profile = JSON_SET(profile, '$.skills[0]', 'PostgreSQL') 
WHERE employee_id = 101; 

六、高级连接技术

1. 交叉连接

-- 生成笛卡尔积 
SELECT e.first_name, d.department_name 
FROM employees e CROSS JOIN departments d; 

2. 自然连接

-- 自动匹配同名列 
SELECT first_name, last_name, department_name 
FROM employees NATURAL JOIN departments; 

3. 使用USING简化连接

-- 当连接列名相同时 
SELECT e.first_name, d.department_name 
FROM employees e JOIN departments d USING (department_id); 

七、事务与锁定查询

1. 事务控制

-- 事务示例 
START TRANSACTION; 
UPDATE accounts SET balance = balance - 100 WHERE id = 1; 
UPDATE accounts SET balance = balance + 100 WHERE id = 2; 
COMMIT;
 -- 或 ROLLBACK; 

2. 锁定查询

-- 锁定行 
SELECT * FROM employees WHERE employee_id = 101 FOR UPDATE; 
-- 共享锁 
SELECT * FROM departments WHERE department_id = 5 LOCK IN SHARE MODE; 

八、性能监控与调优

1. 查询性能分析

-- 开启性能分析 
SET profiling = 1; 
-- 执行查询 
SELECT * FROM employees WHERE salary > 5000; 
-- 查看分析结果 
SHOW PROFILE; SHOW PROFILES; 

2. 索引使用情况

-- 查看索引使用统计 
SELECT * FROM sys.schema_index_statistics WHERE table_schema = 'your_database'; 
-- 查找未使用索引 
SELECT * FROM sys.schema_unused_indexes; 

九、特殊查询场景

1. 递归查询

-- 递归CTE查询组织架构 
WITH RECURSIVE org_hierarchy AS 
( 
-- 基础查询：顶级管理者 
SELECT employee_id, first_name, last_name, manager_id, 1 AS level 
FROM employees WHERE manager_id 
IS NULL UNION ALL 
-- 递归查询：下属员工 
SELECT e.employee_id, e.first_name, e.last_name, e.manager_id, h.level + 1 
FROM employees e JOIN org_hierarchy h ON e.manager_id = h.employee_id
) 

SELECT * FROM org_hierarchy ORDER BY level, employee_id; 

2. 全文搜索

-- 创建全文索引
CREATE FULLTEXT INDEX idx_ft_content ON articles(content); 
-- 全文搜索查询 
SELECT * FROM articles WHERE MATCH(content) 
AGAINST('MySQL performance' IN NATURAL LANGUAGE MODE); 

十、MySQL 8.0新特性

1. 通用表表达式

-- 多个
CTE WITH dept_stats AS 
( 
SELECT department_id, COUNT(*) AS emp_count FROM employees GROUP BY department_id 
), 
high_salary AS 
( SELECT employee_id, salary FROM employees WHERE salary > 8000 ) 

SELECT d.department_id, d.emp_count, COUNT(h.employee_id) AS high_paid_count 
FROM dept_stats d LEFT JOIN high_salary h ON h.employee_id 
IN ( SELECT employee_id FROM employees WHERE department_id = d.department_id ) 
GROUP BY d.department_id, d.emp_count; 

2. 窗口函数增强

-- 计算移动平均 
SELECT date, sales, AVG(sales) 
OVER (ORDER BY date ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) 
AS moving_avg FROM daily_sales; 

以上内容涵盖了MySQL查询的各个方面，从基础查询到高级功能，再到性能优化和特殊场景处理。实际应用中，应根据具体需求和数据特点选择合适的查询方式，并通过EXPLAIN等工具不断优化查询性能。