postgresql SQL SELECT FROM ... AS 带有数据类型说明符？
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SQL SELECT FROM ... AS with data type specifier?
提问by VolkA
I have a problem with an SQL query on Postgresql. This select clause is an example from a lecture on databases:
我在 Postgresql 上的 SQL 查询有问题。这个 select 子句是数据库讲座中的一个例子：
1 select t.CourseNr, t.StudentsPerCourse, g.StudentCount,
2 t.StudentsPerCourse/g.StudentCount as Marketshare
3 from (select CourseNr, count(*) as StudentsPerCourse
4 from taking
5 group by CourseNr) t,
6 (select count(*) as StudentCount
7 from Students) g;
The problem is the Marketshare column in line 2. Both StudentsPerCourse and StudentCount are of type integer.
问题是第 2 行的 Marketshare 列。 StudentsPerCourse 和 StudentCount 都是整数类型。
When using this on my Postgresql database, the Marketshare column is evaluated as an int type, while i would need a float/numeric here. I didn't find any way to specify the data type by searching the Postgresql Documentation on SELECT clauses nor by googling. Is there a (preferably standard SQL) way to specify the column type or am I missing something here?
在我的 Postgresql 数据库上使用它时，Marketshare 列被评估为 int 类型，而我在这里需要一个浮点数/数字。我没有找到通过在 SELECT 子句上搜索 Postgresql 文档或通过谷歌搜索来指定数据类型的任何方法。是否有（最好是标准 SQL）方式来指定列类型，或者我在这里遗漏了什么？
采纳答案by Joel Coehoorn
CAST() one or both of the source columns as a decimal/float/real/double/etc type.
CAST() 作为十进制/浮点数/实数/双精度/等类型的源列之一或两者。
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