How to pivot without knowing fixed columns in T-SQL

Question

I have a table called balance which I wish to pivot, however it is quite difficult since the column names would be labelled 1,2,3 and balances would be sorted b

Answer 1

Demo on db<>fiddle

You can use ROW_NUMBER() to mark the number of values, e.g: 1, 2, 3.

Note that: ORDER BY [Balance] DESC to get the generated value as you wish.

DECLARE 
    @columns NVARCHAR(MAX) = '',
    @sql     NVARCHAR(MAX) = '';


 SELECT Customer, Balance, Col = ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY [Balance] DESC)
 into #b
 FROM #a

SELECT @columns += QUOTENAME(Col) + ','
from (SELECT DISTINCT Col FROM #b) A

-- remove the last comma
SET @columns = LEFT(@columns, LEN(@columns) - 1);


SET @sql = 'SELECT * FROM ( SELECT Customer, Balance, Col FROM  #b) src PIVOT( MAX([Balance]) FOR Col IN ('+ @columns +')) AS pivot_table;';

-- execute the dynamic SQL
EXECUTE sp_executesql @sql;

Output

Updated

Since concatenating strings is undocumented and unreliable. It does not always work as expected. So you should resolve with 2 solutions below

1. Use STRING_AGG (From SQL Server 2017 and late)

SELECT STRING_AGG(QUOTENAME(Col), ', ')
from (SELECT DISTINCT Col FROM #b) A
// Output: [1], [2], [3]

2. Use XML Extensions

DECLARE  @columns NVARCHAR(MAX) = ''
SELECT @columns = (
  SELECT QUOTENAME(Col) + ', '
  FROM (SELECT DISTINCT Col FROM #b) A

  FOR XML PATH(''), TYPE
                   ).value('.','varchar(max)')
SELECT @columns 
// Output: [1], [2], [3],

Thanks @GarethD's comment. Check it out on db<>fiddle