-- --------------------------------------------------
-- mysql 函式

-- SOUNDEX() 轉換為語音值,可以將一個字串轉換為描述其語音表示的字母數字模式
SELECT *
FROM USER
WHERE SOUNDEX(username) = SOUNDEX('rot');

-- NOW()返回當前系統時間
SELECT NOW();

-- AVG() 返回某列的平均值
SELECT AVG(DISTINCT prod_price) AS avg_prod_price
FROM products


-- -------------------------------------
-- ------------------------------------
-- ORDER BY 排序

-- 先按prod_price降序，再按prod_name 升序
SELECT * FROM products
ORDER BY prod_price DESC, prod_name ASC;


-- GROUP BY 分組
-- GROUP BY 可以按一列或多列進行分組
-- GROUP BY 按分組欄位進行排序後，ORDER BY 可以以彙總欄位來進行排序。

SELECT prod_id, prod_name, COUNT(vend_id) AS vend_id
FROM products GROUP BY vend_id;

-- 分組後排序

SELECT prod_id, prod_name, COUNT(vend_id) AS vend_id
FROM products GROUP BY vend_id
ORDER BY prod_id DESC;


-- --------------------------------------------------
/*
HAVING 用於對彙總的 GROUP BY 結果進行過濾。
HAVING 要求存在一個 GROUP BY 子句。
WHERE 和 HAVING 可以在相同的查詢中。
HAVING vs WHERE
WHERE 和 HAVING 都是用於過濾。
HAVING 適用於彙總的組記錄；而 WHERE 適用於單個記錄
*/
SELECT prod_id, prod_name, COUNT(vend_id) AS num
FROM products
WHERE prod_name IS NOT NULL
GROUP BY vend_id
HAVING COUNT(vend_id) >= 2;