合并两个查询并返回可能的值之一

Question

select value1 as value from T1 where id=10;

if id does not exist in T1 - execute another query:

select value2 as value from T2 where id=10;

So, I want to join these queries and return a single value ( value1 or value2 ). Is it possible?

SOLUTION:

My solution:

select ifnull(value1, value2) as value from T1 left join T2 using(id) where id=10;

Answer 1

you can join the query using union

select value1 as value from T1 where id=10 
union 
select value2 as value from T2 where id=10;

as a result you can get any one of the value or both

Answer 2

TRY (tested)

SELECT COALESCE(t1.value1, t2.value2) AS Value FROM t1
INNER JOIN t2 USING(id)
WHERE id=10

this will always check first the table t1 for id=10 , if there is no value then see table t2 for the same id

Quoted FROM

The single result column that replaces two common columns is defined using the coalesce operation. That is, for two t1.a and t2.a the resulting single join column a is defined as a = COALESCE(t1.a, t2.a)

Answer 3

You can join the two queries on the id field and then use the COALESCE function to combine the two resulting fields into the output.

This assumes that you already have a list of IDs to join against, though. Otherwise you're stuck doing a union or full join to get such a list first.

Answer 4

你也可以用这个

select Distinct(s1.id) from sample1 as s1 inner join sample2 as s2;

Answer 5

use union of both

like below :

select t1.id from table1 as t1  where id=10
union 
select t2.id from table2 as t2  where id=10