一、列名/表名更改
列名更改
AS關鍵字是可選的，因此您可以在語句中將其省略
SELECT
[column_1 | expression] AS descriptive_name
FROM table_name;
選擇員工的名字和姓氏，使用CONCAT_WS()函數將名字和姓氏連接為全名
SELECT
CONCAT_WS(', ', lastName, firstname) AS `Full name`
FROM
employees;
表名更改
table_name AS table_alias
將employees表別名更改為e：
SELECT * FROM employees e;
為表分配別名后，您可以使用以下語法引用表列：
table_alias.column_name
SELECT
e.firstName,
e.lastName
FROM
employees e
ORDER BY e.firstName;
SELECT
customerName,
COUNT(o.orderNumber) total
FROM
customers c
INNER JOIN orders o ON c.customerNumber = o.customerNumber
GROUP BY
customerName
ORDER BY
total DESC;
二、INNER JOIN 內連接,等值連接
SELECT
select_list
FROM t1
INNER JOIN t2 ON join_condition1
INNER JOIN t3 ON join_condition2
...;
示例：
查詢 產品的號碼、產品名、產品描述
SELECT
productCode,
productName,
textDescription
FROM
products t1
INNER JOIN productlines t2
ON t1.productline = t2.productline;
由於兩個表的聯接列都具有相同的名稱 productline，因此可以使用以下USING
SELECT
productCode,
productName,
textDescription
FROM
products
INNER JOIN productlines USING (productline);
示例2：
從orders和orderdetails表中返回來自和表的訂單號，訂單狀態和總銷售額：INNER JOIN``GROUP BY
SELECT
t1.orderNumber,
t1.status,
SUM(quantityOrdered * priceEach) total
FROM
orders t1
INNER JOIN orderdetails t2
ON t1.orderNumber = t2.orderNumber
GROUP BY orderNumber;
INNER JOIN–連接三個表示例：
SELECT
orderNumber,
orderDate,
orderLineNumber,
productName,
quantityOrdered,
priceEach
FROM
orders
INNER JOIN
orderdetails USING (orderNumber)
INNER JOIN
products USING (productCode)
ORDER BY
orderNumber,
orderLineNumber;
INNER JOIN`使用其他運算符
SELECT
orderNumber,
productName,
msrp,
priceEach
FROM
products p
INNER JOIN orderdetails o
ON p.productcode = o.productcode
AND p.msrp > o.priceEach
WHERE
p.productcode = 'S10_1678';
三、LEFT JOIN 左連接
LEFT JOIN返回左表中的所有行，而不管左表中的行是否與右表中的行匹配。如果沒有匹配項，則右表中該行的列將包含NULL
SELECT
select_list
FROM
t1
LEFT JOIN t2 ON
join_condition;
SELECT
c.customerNumber,
customerName,
orderNumber,
status
FROM
customers c
LEFT JOIN orders o
ON c.customerNumber = o.customerNumber;
LEFT JOIN 查找不匹配的行
SELECT
c.customerNumber,
c.customerName,
o.orderNumber,
o.status
FROM
customers c
LEFT JOIN orders o
ON c.customerNumber = o.customerNumber
WHERE
orderNumber IS NULL;
對INNER JOIN子句，該子句中on的使用 可以等效WHERE 但在LEFT JOIN不等效
SELECT
o.orderNumber,
customerNumber,
productCode
FROM
orders o
LEFT JOIN orderDetails
USING (orderNumber)
WHERE
orderNumber = 10123;
SELECT
o.orderNumber,
customerNumber,
productCode
FROM
orders o
LEFT JOIN orderDetails d
ON o.orderNumber = d.orderNumber AND
o.orderNumber = 10123;
-Condition-in-WHERE-clause.png)
四、RIGHT JOIN 右鏈接
RIGHT JOIN返回右表中的所有行，而不管右表中的行是否與左表中的行匹配。如果沒有匹配項，則右左表中該行的列將包含NULL
五、CROSS JOIN 交叉連接
從聯接表中返回行的笛卡爾積
SELECT * FROM t1
CROSS JOIN t2;
獲取所有商店和產品的組合
SELECT
store_name, product_name
FROM
stores AS a
CROSS JOIN
products AS b;
https://www.mysqltutorial.org/mysql-cross-join/
六、SELF JOIN 自連接
SELECT
CONCAT(m.lastName, ', ', m.firstName) AS Manager,
CONCAT(e.lastName, ', ', e.firstName) AS 'Direct report'
FROM
employees e
INNER JOIN employees m ON
m.employeeNumber = e.reportsTo
ORDER BY
Manager;
SELECT
IFNULL(CONCAT(m.lastname, ', ', m.firstname),
'Top Manager') AS 'Manager',
CONCAT(e.lastname, ', ', e.firstname) AS 'Direct report'
FROM
employees e
LEFT JOIN employees m ON
m.employeeNumber = e.reportsto
ORDER BY
manager DESC;
顯示位於同一城市的客戶列表
SELECT
c1.city,
c1.customerName,
c2.customerName
FROM
customers c1
INNER JOIN customers c2 ON
c1.city = c2.city
AND c1.customername > c2.customerName
ORDER BY
c1.city;
c1.city = c2.city確保兩個客戶都在同一個城市。c.customerName > c2.customerName確保不包括同一位客戶。