如何将逻辑运算符应用于 2 Select 语句？ [复制]

Question

我有 4 个名为 A, B, C, D 的表。我有2 conditons to get the data

Condition 1

Select *
From A, B, C
Where //All 3 tables are joined with conditions
limit 1; // Display 1 data at a time

Condition 2

Select *
From A, B, D
Where //All 3 tables are joined with conditions
limit 1; // Display 1 data at a time

我是什么looking for is to combine above 2 Select Statement 这样

If Data Exists in Condition 1 然后Display Data 或 Display Data from condition 2

如何实现上述功能？

Answer 1

你可以这样做

if exists(/*First query*/)
begin

/*FirstQuery*/

end

else

begin

/*Second Query*/

end

Answer 2

我在想

Select *
From A, B, C
Where //All 3 tables are joined with conditions
limit 1 // Display 1 data at a time
UNION
Select *
From A, B, D
Where not exists(Select *
From A, B, C
Where //All 3 tables are joined with conditions)
limit 1