只是因为它略有不同:

SELECT 'table_1' AS table_name, COUNT(*) FROM table_1
UNION
SELECT 'table_2' AS table_name, COUNT(*) FROM table_2
UNION
SELECT 'table_3' AS table_name, COUNT(*) FROM table_3

它给出了转置的答案(每个表一行而不是一列)，否则我不认为它有多大不同。我认为在性能方面，它们应该是相等的。

选择 (select count() from tab1 where field like 'value') + (select count() from tab2 where field like 'value') 数

    select 
    t1.Count_1,t2.Count_2
    from 
(SELECT count(1) as Count_1 FROM tab1) as t1, 
(SELECT count(1) as Count_2 FROM tab2) as t2

SELECT (SELECT COUNT(*) FROM table1) + (SELECT COUNT(*) FROM table2) FROM dual;

作为附加信息，要在SQL Server中完成同样的事情，您只需要删除查询的“FROM dual”部分。

select (select count(*) from tab1) count_1, (select count(*) from tab2) count_2 from dual;