MySQL检查是否存在多行

Question

Here you can find how to check row existance:
SELECT EXISTS(SELECT 1 FROM table1 WHERE some_condition);

如何从表格中有效地存在多行：

SELECT EXISTS(SELECT 1 FROM table1 WHERE key = 0);
SELECT EXISTS(SELECT 1 FROM table1 WHERE key = 2);

来自表格：

key,username
0,foo
1,bar
2,boo

只有在找到两个行（带有键0和2）时才返回正数？

Answer 1

SELECT count(distinct key) = 2  FROM table1 WHERE key in (0, 2) 

Answer 2

SELECT sum(`key` = 0) as key0_count,
       sum(`key` = 2) as key2_count 
FROM your_table