SQL Server 2017

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CREATE TABLE t(i INT, c VARCHAR(10));
INSERT INTO t(i,c) VALUES(1, 'a'), (2, 'b'), (3, 'c'), (4, 'd'), (5, 'e')
5 rows affected

SELECT * FROM t;


i
c


1
a

2
b

3
c

4
d

5
e


SELECT STRING_AGG(c,(CASE WHEN i%2=0 THEN ' OR ' ELSE ' AND ' END)) WITHIN GROUP (ORDER BY i) AS r
FROM t;
Msg 8733 Level 16 State 1 Line 1
Separator parameter for STRING_AGG must be a string literal or variable.