59 lines
2.1 KiB
Markdown
59 lines
2.1 KiB
Markdown
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---
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title: SQL COUNT Aggregate Function
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localeTitle: SQL COUNT聚合函数
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---
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## SQL COUNT聚合函数
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COUNT运算符通常与GROUP BY子句结合使用。它是SQL“聚合”函数之一，包括AVG（平均值）和SUM。
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此函数将计算行数并将该计数作为结果集中的列返回。
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以下是您将使用COUNT的示例：
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* 计算表中的所有行（不需要按组）
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* 计算数据子集的总数（需要语句的Group By部分）
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作为参考，这是我们的示例学生数据库中所有行的当前数据。
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```sql
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select studentID, FullName, programOfStudy, sat_score from student; -- all records with fields of interest
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```
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此SQL语句提供所有行的计数。请注意，您可以使用“AS”为生成的COUNT列指定名称。
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```sql
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select count(*) AS studentCount from student; -- count of all records
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```
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在这里，我们得到每个学习领域的学生数量。
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```sql
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select studentID, FullName, count(*) AS studentCount from the student table with a group by programOfStudy;
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```
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在这里，我们得到具有相同SAT分数的学生数。
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```sql
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select studentID, FullName, count(*) AS studentCount from the student table with a group by sat_score;
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```
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以下是使用广告系列基金表的示例。这是2016年美国总统竞选期间每笔交易的美元总额和每个政党的捐款数量。
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```sql
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select Specific_Party, Election_Year, format(sum(Total_$),2) AS contribution$Total, count(*) AS numberOfContributions
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from combined_party_data
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group by Specific_Party,Election_Year
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having Election_Year = 2016;
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```
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与所有这些事情一样，还有更多内容，所以请查看数据库管理员的手册，并自己尝试不同的测试。
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