计数与总计

SQL Server 中级

Ginger Grant

Instructor

用计数查看总量

SELECT COUNT(*) FROM Incidents

+-----------------+
|(No column name) |
+-----------------+
|6452             | 
+-----------------+

COUNT 与 DISTINCT

COUNT(DISTINCT COLUMN_NAME)

T-SQL 中的 COUNT 与 DISTINCT（I）

SELECT COUNT(DISTINCT Country) AS Countries
FROM Incidents

+----------------+
|Countries       |
+-----------------+
|3               |
+-----------------+

T-SQL 中的 COUNT 与 DISTINCT（II）

SELECT COUNT(DISTINCT Country) AS Countries,
COUNT(DISTINCT City) AS Cities 
FROM Incidents

+----------------+-------------+
|Countries       | Cities      |
+-----------------+-------------+
|3               | 3566        |
+-----------------+-------------+

COUNT 聚合

GROUP BY 与 COUNT() 的用法与其他聚合（如 AVG()、MIN()、MAX()) 相同
用 ORDER BY 排序
- ASC 返回从小到大（默认）
- DESC 返回从大到小

T-SQL 中的 COUNT 与 GROUP BY

-- 按 Country 分组统计行数
SELECT COUNT(*) AS TotalRowsbyCountry, Country
FROM Incidents
GROUP BY Country

+----------------------+-----------------+
|TotalRowsbyCountry   | Country         |
+----------------------+-----------------+
|5452                 |us               |
|750                  |NULL             |
|249                  |ca               |
|1                    |gb               |
+----------------------+-----------------+

T-SQL 中的 COUNT、GROUP BY 与 ORDER BY（I）

-- 按 Country 分组统计行数
SELECT COUNT(*) AS TotalRowsbyCountry, Country
FROM Incidents
GROUP BY Country 
ORDER BY Country ASC

+----------------------+-----------------+
|TotalRowsbyCountry   | Country         |
+----------------------+-----------------+
|750                  |NULL             |
|249                  |ca               |
|1                    |gb               |
|5452                 |us               |
+----------------------+-----------------+

T-SQL 中的 COUNT、GROUP BY 与 ORDER BY（II）

-- 按 Country 分组统计行数
SELECT COUNT(*) AS TotalRowsbyCountry, Country
FROM Incidents
GROUP BY Country 
ORDER BY Country DESC

+----------------------+-----------------+
|TotalRowsbyCountry   | Country         |
+----------------------+-----------------+
|5452                 |us               |
|1                    |gb               |
|249                  |ca               |
|750                  |NULL             |
+----------------------+-----------------+

用 SUM 求列总计

SUM() 计算列值的总和
用法与其他聚合相同
结合 GROUP BY 可按指定列求小计

在 T-SQL 中求列和

-- 按 Country 求值的小计
SELECT SUM(DurationSeconds) AS TotalDuration, Country
FROM Incidents
GROUP BY Country

+----------+--------------------+
|Country   |TotalDuration       |
+----------+--------------------+
|us        |17024946.750001565  |
|null      |18859192.800000012  |
|ca        |200975              |
|gb        |120                 |
+----------+--------------------+

Passons à la pratique !

SQL Server 中级