MySQL数据库提供了多种查询命令，用于从数据库表中检索数据，以下是一些常用的查询命令及其简要说明：

1. SELECT语句

基本语法：

SELECT column1, column2, ...
FROM table_name;

示例：

SELECT first_name, last_name
FROM employees;

2. WHERE子句

基本语法：

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

示例：

SELECT first_name, last_name
FROM employees
WHERE salary > 50000;

3. ORDER BY子句

基本语法：

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;

4. GROUP BY子句

基本语法：

SELECT column1, column2, ...
FROM table_name
GROUP BY column1, column2, ...;

示例：

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

5. HAVING子句

基本语法：

SELECT column1, column2, ...
FROM table_name
GROUP BY column1, column2, ...
HAVING condition;

示例：

SELECT department, COUNT(*)
FROM employees
GROUP BY department
HAVING COUNT(*) > 10;

6. JOIN操作

基本语法：

SELECT column1, column2, ...
FROM table1
JOIN table2 ON table1.column = table2.column;

示例：

SELECT employees.first_name, employees.last_name, departments.department_name
FROM employees
JOIN departments ON employees.department_id = departments.department_id;

这些是MySQL中常用的查询命令，可以根据具体需求进行组合使用。