SQL查詢語句（三）—— 分組數據

一、GROUP BY 分組

SELECT 
    c1, c2,..., cn, aggregate_function(ci)
FROM
    table
WHERE
    where_conditions
GROUP BY c1 , c2,...,cn;

語句執行順序

將訂單狀態的值分組

SELECT 
    status
FROM
    orders
GROUP BY status;

與聚合函數一起使用

每種狀態下的訂單數量

SELECT 
    status, COUNT(*)
FROM
    orders
GROUP BY status;

所有訂單的總金額

SELECT 
    status, 
    SUM(quantityOrdered * priceEach) AS amount
FROM
    orders
INNER JOIN orderdetails 
    USING (orderNumber)
GROUP BY 
    status;

查詢返回訂單號和每個訂單的總金額

SELECT 
    orderNumber,
    SUM(quantityOrdered * priceEach) AS total
FROM
    orderdetails
GROUP BY 
    orderNumber;

查詢獲取每年的總銷售額

SELECT 
    YEAR(orderDate) AS year,
    SUM(quantityOrdered * priceEach) AS total
FROM
    orders
INNER JOIN orderdetails 
    USING (orderNumber)
WHERE
    status = 'Shipped'
GROUP BY 
    YEAR(orderDate);

過濾由GROUP BY子句返回的組，使用having

SELECT 
    YEAR(orderDate) AS year,
    SUM(quantityOrdered * priceEach) AS total
FROM
    orders
INNER JOIN orderdetails 
    USING (orderNumber)
WHERE
    status = 'Shipped'
GROUP BY 
    year
HAVING 
    year > 2003;

標准SQL不允許您在GROUP BY子句中使用別名，但是MySQL支持這一點

SELECT 
    YEAR(orderDate) AS year, 
    COUNT(orderNumber)
FROM
    orders
GROUP BY 
    year;

MySQL還允許您按升序或降序對組進行排序，而標准SQL則不允許

SELECT 
    status, 
    COUNT(*)
FROM
    orders
GROUP BY 
    status DESC;

二、HAVING

用於為一組行或集合指定過濾條件

SELECT 
    select_list
FROM 
    table_name
WHERE 
    search_condition
GROUP BY 
    group_by_expression
HAVING 
    group_condition;

語句執行順序

獲取訂單號，每個訂單售出的商品數以及每個商品的總銷售額

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY ordernumber;

訂單的總銷售額大於1000

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY 
   ordernumber
HAVING 
   total > 1000;

HAVING使用OR 或者 AND

總金額大於1000且包含多個600項目的訂單：

SELECT 
    ordernumber,
    SUM(quantityOrdered) AS itemsCount,
    SUM(priceeach*quantityOrdered) AS total
FROM
    orderdetails
GROUP BY ordernumber
HAVING 
    total > 1000 AND 
    itemsCount > 600;

參考資料

basic-mysql-tutorial