mysql - SELECT SUM(1) FROM ( SELECT '0' as R FROM dual UNION ALL SELECT '1' as R FROM dual )

Question

为什么下面的查询给出值 2 而不是我期望的 1？

SELECT SUM(1) FROM (
SELECT  '0' as R FROM dual
UNION 
SELECT  '1' as R FROM dual
)

但是这个查询满足预期？

SELECT SUM(R) FROM (
SELECT  '0' as R FROM dual
UNION 
SELECT  '1' as R FROM dual
)

Answer 1

因为在第一个查询中，您对选择中的任何行的每次出现求和 1，而对于查询 1，您对选择中的每一行的“R”值求和。