以下为 SQL 中聚合函数、分组查询及汇总的用法解析。需要相关数据可私信。
学习内容
- 聚合函数
- COUNT()：统计行数，忽略空值；COUNT(*) 计数所有行。
- SUM()：计算总和。
- AVG()：求平均值。
- MAX()：返回最大值。
- MIN()：返回最小值。
- GROUP_CONCAT()：合并多行为字符串。
- 处理列数据，返回单一结果。
- 常见函数：
- 支持表达式(如 SUM(invoice_total * 1.4))，可用于日期。
- DISTINCT 去除重复值。
-
GROUP BY 子句
-
- 按指定列分组，结合聚合函数计算每组结果。
- 顺序：SELECT → FROM → WHERE → GROUP BY → ORDER BY。
- 多列分组用逗号分隔，无需括号。
-
HAVING 子句
-
- 过滤分组后的结果，解决 WHERE 无法处理聚合值的问题。
- 置于 GROUP BY 后，支持多条件。
-
WITH ROLLUP
-
- 在 GROUP BY 后添加，生成分组汇总行。
- 多列分组时，逐级汇总至总计。
示例代码与讲解
1. 聚合函数
SELECT MAX(invoice_total)
FROM invoices;
- 返回 invoice_total 最大值。
SELECT
MAX(invoice_total) AS highest,
MIN(invoice_total) AS lowest,
AVG(invoice_total) AS AVG,
SUM(invoice_total * 1.4) AS SUM,
COUNT(invoice_total) AS COUNT
FROM invoices;
- 多聚合查询，含表达式，重命名结果为五列。
SELECT
MAX(payment_date) AS highest,
MIN(payment_date) AS lowest,
COUNT(payment_date) AS COUNT
FROM invoices;
- 应用于日期，COUNT 不计空值。
SELECT COUNT(*) AS number_invoices
FROM invoices;
- 统计所有行，含空值。
SELECT
MAX(payment_date) AS highest,
MIN(payment_date) AS lowest,
COUNT(payment_date) AS COUNT
FROM invoices
WHERE payment_date >= '2018-01-01';
- 筛选后聚合。
SELECT COUNT(DISTINCT client_id) AS number_client
FROM invoices;
- 统计唯一 client_id，避免重复计数。
2. GROUP BY
SELECT client_id, SUM(invoice_total) AS Total_sales
FROM invoices
GROUP BY client_id;
- 按 client_id 分组，计算每组销售总额。
SELECT client_id, SUM(invoice_total) AS Total_sales
FROM invoices
WHERE invoice_date >= '2019-07-01'
GROUP BY client_id
ORDER BY Total_sales DESC;
- 筛选后分组，按总额降序。
SELECT c.city, c.state, SUM(i.invoice_total) AS Total_sales
FROM invoices i
JOIN clients c USING (client_id)
GROUP BY c.state, c.city;
- 连接表后，按州和城市分组，逗号分隔条件。
3. HAVING
SELECT client_id, SUM(invoice_total) AS Total_sales
FROM invoices
GROUP BY client_id
HAVING Total_sales >= 800;
- 筛选销售总额 ≥ 800 的组。
SELECT client_id, SUM(invoice_total) AS Total_sales, COUNT(*) AS number_invoices
FROM invoices
GROUP BY client_id
HAVING Total_sales >= 800 AND number_invoices > 5;
- 多条件筛选：总额 ≥ 800 且发票数 > 5。
4. WITH ROLLUP
SELECT client_id, SUM(invoice_total) AS Total_sales
FROM invoices
GROUP BY client_id WITH ROLLUP;
- 添加汇总行，显示所有 client_id 的总计。
SELECT state, city, SUM(invoice_total) AS Total_sales
FROM invoices
JOIN clients c USING (client_id)
GROUP BY state, city WITH ROLLUP;
- 按州和城市分组，逐级汇总至总计。
作业
1. 聚合与 UNION
SELECT
'First half of 2019' AS date_range,
SUM(invoice_total) AS Total_sales,
SUM(payment_total) AS Total_payments,
SUM(invoice_total) - SUM(payment_total) AS what_we_expect
FROM invoices
WHERE invoice_date BETWEEN '2019-01-01' AND '2019-06-30'
UNION
SELECT
'Second half of 2019' AS date_range,
SUM(invoice_total) AS Total_sales,
SUM(payment_total) AS Total_payments,
SUM(invoice_total) - SUM(payment_total) AS what_we_expect
FROM invoices
WHERE invoice_date BETWEEN '2019-07-01' AND '2019-12-31'
UNION
SELECT
'Total' AS date_range,
SUM(invoice_total) AS Total_sales,
SUM(payment_total) AS Total_payments,
SUM(invoice_total) - SUM(payment_total) AS what_we_expect
FROM invoices
WHERE invoice_date BETWEEN '2019-01-01' AND '2019-12-31';
- 计算 2019 年上下半年及全年的销售、付款和预期差额。
2. 分组查询
SELECT date, pm.name AS payment_method, SUM(amount) AS Total_payments
FROM payments p
JOIN payment_methods pm ON p.payment_method = pm.payment_method_id
GROUP BY date, pm.name
ORDER BY date;
- 按日期和支付方式分组，计算付款总额。
3. HAVING 筛选
USE sql_store;
SELECT c.customer_id, SUM(oi.quantity * oi.unit_price) AS total_buy
FROM customers c
JOIN orders o USING (customer_id)
JOIN order_items oi USING (order_id)
WHERE c.state = 'VA'
GROUP BY c.customer_id
HAVING total_buy >= 100;
- 查询 VA 州客户总购买额，筛选 ≥ 100 的结果。
4. ROLLUP 汇总
USE sql_invoicing;
SELECT pm.name, SUM(p.amount) AS SUM
FROM payments p
JOIN payment_methods pm ON p.payment_method = pm.payment_method_id
GROUP BY pm.name WITH ROLLUP;
- 按支付方式分组，含总计。
总结
本次解析了聚合函数(COUNT、SUM 等)、GROUP BY 分组、HAVING 过滤及 ROLLUP 汇总。基于 sql_invoicing 和 sql_store 数据库。