如何在case语句中使用子查询？

Question

I have two tables .

table1

t1id var1 var2
1    xxx   ccc
2    ccc   ddd
3    eee   bbb

table2

t2id attribute 
1    lll  
3    ggg
4    ggg
5    lll

I need to select * from table1, and concatenate t1id with attribute if t1id=t2id

Here is what I tried

SEL CASE WHEN (t1id IN (SELECT t2id FROM table2) ) 
    THEN t1id || attribute 
    ELSE t1id END AS t1idModified,
var1 , var2
FROM table1,table2

However, it did not work . How can I use a subquery within case?

Answer 1

Even if this syntax was allowed it would not return the expected result.

What's the datatype of t1id , numeric or string?

You can rewrite it as an Outer Join (return a row even if there's no match) and a COALESCE (concat the attribute or ''):

SELECT
   TRIM(t1.t1id) || (COALESCE(t2.ATTRIBUTE, '')) AS t1idModified,
   var1, var2
FROM table1 AS t1
LEFT JOIN table2 AS t2
ON t1.t1id = t2.t2id