MySQL是一種關系型數據庫管理系統，是最流行的開源數據庫。

在MySQL中，查詢是最基本的操作之一。下面是一些MySQL查詢的實例：

-- 查詢表中所有的數據
SELECT * FROM table_name;
-- 按條件查詢數據
SELECT column_name FROM table_name WHERE condition;
-- 按條件查詢并按指定列排序
SELECT column_name FROM table_name WHERE condition ORDER BY column_name ASC/DESC;
-- 查詢某列的不同值
SELECT DISTINCT column_name FROM table_name;
-- 使用聚合函數計算數據
SELECT AVG(column_name) FROM table_name WHERE condition;
SELECT COUNT(*) FROM table_name WHERE condition;
SELECT MAX(column_name) FROM table_name WHERE condition;
SELECT MIN(column_name) FROM table_name WHERE condition;
SELECT SUM(column_name) FROM table_name WHERE condition;
-- 多表查詢
SELECT column_name FROM table1_name, table2_name WHERE condition;
-- 使用子查詢查詢數據
SELECT column_name FROM table_name WHERE column_name IN 
(SELECT column_name FROM another_table WHERE condition);
-- 使用JOIN關鍵字連接多張表
SELECT column_name FROM table1_name JOIN table2_name ON condition;
-- 對查詢結果進行分組
SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name;
-- 使用HAVING關鍵字對分組數據進行過濾
SELECT column_name, COUNT(*), AVG(column_name) FROM table_name GROUP BY column_name HAVING COUNT(*) >1;
-- 使用LIMIT關鍵字限制查詢結果數量
SELECT column_name FROM table_name LIMIT 20;