UNION

SQL UNION

下面的SQL語句從product和orderdetail表中選取所有不同的pCode(只有不同值)

SELECT pCode FROM product
UNION
SELECT pCode FROM orderdetail
ORDER BY pCode;

SQL UNION ALL

下面的SQL語句從product和orderdetail表中選取所有的pCode（也有重複值）

SELECT pCode FROM product
UNION ALL
SELECT pCode FROM orderdetail
ORDER BY pCode;
 

帶有 WHERE 的 SQL UNION ALL

下面的SQL語句從product和orderdetail表中選取所有的pCode=101的值（也有重複值）

SELECT pCode, pName FROM product 
WHERE pCode=101 
UNION ALL
SELECT pCode, qty FROM orderdetail 
WHERE pCode=101  
ORDER BY pCode;

排序

ORDER BY子句

ASC升序，DESC降序

SELECT * FROM product ORDER BY cost ASC;
SELECT *
 
 FROM product ORDER BY cost DESC;

分組

GROUP BY語句

統計每種型別的球有多少條記錄

SELECT pType, COUNT(*) FROM product
GROUP BY pType;

WITH ROLLUP

統計pType中相同型別的球cost的總數，命名為cost_count

SELECT pType, SUM(cost) as cost_count
FROM product 
GROUP BY pType WITH ROLLUP;

執行完後最後一行是NULL，取代NULL的語句coalesce(a,b,c)

引數說明：如果a==null,則選擇b；如果b==null,則選擇c；如果a!=null,則選擇a；如果a b c 都為null ，則返回為null（沒意義）。

SELECT coalesce(pType, '總數'), SUM(cost) as cost_count
FROM product 
GROUP BY pType WITH ROLLUP;

連線

INNER JOIN

SELECT a.pId, a.pCode, a.pType, b.qty 
FROM product a INNER JOIN orderdetail b
ON a.pCode = b.pCode;

使用while子句

SELECT a.pId, a.pCode, a.pType, b.qty 
FROM product a, orderdetail b
WHERE a.pCode = b.pCode;

LEFT JOIN
MySQL left join 與join 有所不同。 LEFT JOIN 會讀取左邊資料表的全部資料，即便右邊表無對應資料。

SELECT a.pId, a.pCode, a.pType, b.qty 
FROM product a LEFT JOIN orderdetail b
ON a.pCode = b.pCode;

RIGHT JOIN
RIGHT JOIN 會讀取右邊資料表的全部資料，即便左邊邊表無對應資料。

SELECT a.pId, a.pCode, a.pType, b.qty 
FROM product a RIGHT JOIN orderdetail b
ON a.pCode = b.pCode;

NULL值處理

查詢資料表orderdetail 中discount列是否為NULL，必須使用 IS NULL和 IS NOT NULL，如下例項：

SELECT * FROM orderdetail WHERE discount IS NOT NULL;
SELECT * FROM orderdetail WHERE discount IS NULL;

正則表示式

之前的LIKE是用來進行模糊匹配

使用REGEXP操作符來進行正則表示式匹配。

# 以"足"字開頭的所有資料
SELECT pType FROM product WHERE pType REGEXP '^足';
# 以"類"字開頭的所有資料
SELECT pType FROM product WHERE pType REGEXP '類$';
# 欄位中含有"球"字的所有資料
SELECT pType FROM product WHERE pType REGEXP '球';
# 欄位中含有"球"或者"泳"字且以"類"字的所有資料
SELECT pType FROM product WHERE pType REGEXP '[球泳]|類&';