SQL Group By - 単一の列から複数の集計列を生成する

次のような相関サブクエリを使用してそれを行うことができます:

SELECT 
  Company, 
  Date, 
  (SELECT COUNT(*) FROM MyTable AS T1 
     WHERE T1.Flag='Y' AND T1.Company=T2.Company AND T1.Date=T2.Date) AS Count_Y,
  (SELECT COUNT(*) FROM MyTable AS T1 
     WHERE T1.Flag='N' AND T1.Company=T2.Company AND T1.Date=T2.Date) AS Count_N
FROM MyTable AS T2
GROUP BY Company, Date
  

 もっと簡潔にすることもできますが、おそらく SUM トリック:
 
SELECT 
  Company, 
  Date, 
  SUM(CASE WHEN Flag='Y' THEN 1 ELSE 0 END) AS Count_Y,
  SUM(CASE WHEN Flag='N' THEN 1 ELSE 0 END) AS Count_N,
FROM MyTable
GROUP BY Company, Date
  

 Oracle/PLSQL では、DECODE 関数を使用して CASE を置き換えることができます さらに簡潔に:
 
SELECT 
  Company, 
  Date, 
  SUM(DECODE(Flag,'Y',1,0)) AS Count_Y,
  SUM(DECODE(Flag,'N',1,0)) AS Count_N,
FROM MyTable
GROUP BY Company, Date