SQL常用语句--超实用！！！
数据操作：
1.插入数据(数据需要一一对应)
INSERT INTO employees (employee_id,name,age,department_id)
VALUES(1,'John Doe',30,5);
2.更新数据
UPDATE employees
SET name = 'Jane Doe'
WHERE employee_id=1;
3.删除数据
DELETE FROM employees WHERE employee_id = 1;
4.查询所有数据
SELETE * FROM employees;
查询操作：
5.查询特定列
SELETE name,age
FROM employees;
6.条件查询
SELETE *
FROM employees
WHERE age>30;
7.限制查询结果数量
SELETE *
FROM employees
LIMIT 10;
8.查询排序
SELETE *
FROM employees
ORDER BY age DESC;
9.分组统计
SELECT department_id,COUNT(*) AS num_employees
FROM employees
GROUP BY department_id;
10.连接查询
SELECT employees.name,departments.name
FROM employees
JOIN departments
ON employees.department_id = departments.department_id;
11.左连接查询
SELECT employees.name,departments.name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;
12.子查询(括号内优先执行)
SELECT name FROM employees
WHERE department_id
IN(SELECT department_id FROM departments WHERE name = 'IT');
计算操作：
13.计算总和
SELECT SUM(salary) FROM employees;
14.计算平均值
SELECT AVG(salary) FROM employees;
15.计算最大值
SELECT MAX(salary) FROM employees;
16.计算最小值
SELECT MIN(salary) FROM employees;
17.计数
SELECT COUNT(*) FROM employees;
其他操作：
18.使用别名
SELECT COUNT(*) AS total_employees
FROM employees;
19.去重查询
SELECT DISTINCT department_id FROM employees;
20.使用条件函数
SELECT name,
CASE WHEN age >= 18
THEN 'Adult'
ELSE 'Minor'
END AS status
FROM employees;
21.使用LIKE进行模糊查询
SELECT * FORM employees
WHERE name = '%J%';
22.使用BETWEEN查询范围
SELECT * FROM employees
WHERE age BETWEEN 25 AND 35;
23.使用IS NULL检查空值
SELECT * FROM employees
WHERE department_id IS NULL;
24.使用IS NOT NULL检查非空值
SELECT * FROM employees
WHERE department_id IS NOT NULL;
