Create table table1(id int, name varchar(20), city varchar(20))

Declare @a as varchar(4000)
SET @a = 'xxx '
SELECT @a = @a + '['+COLUMN_NAME+'] ' FROM INFORMATION_SCHEMA.COLUMNS
SELECT @a

Declare @a as varchar(4000)
SET @a = 'xxx '
SELECT 'b' = @a + '['+COLUMN_NAME+'] ' FROM INFORMATION_SCHEMA.COLUMNS

Declare @a as varchar(4000)
SET @a = 'xxx '
SELECT Top 1 @a = @a + '['+COLUMN_NAME+'] ' FROM INFORMATION_SCHEMA.COLUMNS
SELECT @a