mysql查询几个列的数据

SELECT name, position FROM employees;

SELECT e.name, e.position, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.id;

SELECT name, position
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

SELECT department_name, COUNT(*) as employee_count
FROM employees e
JOIN departments d ON e.department_id = d.id
GROUP BY department_name;

SELECT name, position FROM employees WHERE id = 1;

-- 添加索引
ALTER TABLE employees ADD INDEX idx_name (name);

-- 优化查询
SELECT name, position FROM employees WHERE name LIKE 'John%';

SELECT name, positon FROM employees; -- 错误
SELECT name, position FROM employees; -- 正确