计算几个表并在SQL中以多行返回数据

Question

我正在尝试学习SQL,目前我正在学习COUNT函数.我想测试从多个表中提取数据,我想返回一个结果集,如:

| tablename | row_count |
| Computers | 2000      |
| Buildings | 37        |

到目前为止,我还没有找到一种方法来获取信息并制作这样的视图.

Answer 1

使用union和带有(常量)标签的count(*):

select 'Computers' as tablename, count(*) as row_count from Computers
union all
select 'Buildings' as tablename, count(*) as row_count from Buildings

请注意,使用union all(而不仅仅是union)意味着返回的行将保持在查询中选择的顺序.