SQL Server 2019

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DECLARE @x AS NVARCHAR(MAX)
DECLARE @y AS NVARCHAR(MAX) = '_10 chars_'
DECLARE @z AS VARCHAR(4200) = replicate('_', 4200)

SET @x = @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 4200'

SET @x = @z + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 8400 = 4200 * 2'

SET @x = @z + @z + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 12600 = 4200 * 3'

SET @x = @z + @y
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 4210 = 4200 + 10'

SET @x = @z + @z + @y
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 8410 = 4200 * 2 + 10'

SET @x = @z + @z + @z + @y
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 12610 = 4200 * 3 + 10'

SET @x = @y + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 4210 = 10 + 4200'

SET @x = @y + @z + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 8410 = 10 + 4200 * 2'

SET @x = @y + @z + @z + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 12610 = 10 + 4200 * 3'

SET @x = @z + @z + @z + @y + @z + @z + @z
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 25210 = 4200 * 3 + 10 + 4200 * 3'

SET @x = CONCAT(@z, @z, @z, @y, @z, @z, @z);
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 25210 = 4200 * 3 + 10 + 4200 * 3'

SET @x = CAST(@z AS nvarchar(max)) + CAST(@z AS nvarchar(max)) + CAST(@z AS nvarchar(max)) + @y + CAST(@z AS nvarchar(max)) + CAST(@z AS nvarchar(max)) + CAST(@z AS nvarchar(max))
print CAST(len(@x) AS NVARCHAR(MAX)) + '    Expected 25210 = 4200 * 3 + 10 + 4200 * 3'

4200    Expected 4200
8000    Expected 8400 = 4200 * 2
8000    Expected 12600 = 4200 * 3
4010    Expected 4210 = 4200 + 10
4010    Expected 8410 = 4200 * 2 + 10
4010    Expected 12610 = 4200 * 3 + 10
4010    Expected 4210 = 10 + 4200
8010    Expected 8410 = 10 + 4200 * 2
12010    Expected 12610 = 10 + 4200 * 3
16010    Expected 25210 = 4200 * 3 + 10 + 4200 * 3
25210    Expected 25210 = 4200 * 3 + 10 + 4200 * 3
25210    Expected 25210 = 4200 * 3 + 10 + 4200 * 3