Du kan bruge et fælles tabeludtryk til at give hver køber en ordre hos leverandøren, og derefter bare lave en almindelig sag for at sætte dem i kolonner;
WITH cte AS (
SELECT supplier, buyer,
ROW_NUMBER() OVER (PARTITION BY supplier ORDER BY buyer) rn
FROM Table1
)
SELECT supplier,
MAX(CASE WHEN rn=1 THEN buyer END) buyer1,
MAX(CASE WHEN rn=2 THEN buyer END) buyer2,
MAX(CASE WHEN rn=3 THEN buyer END) buyer3
FROM cte
GROUP BY supplier;