同じ列で異なる値がカウントされます

COUNT関数内でCASEまたはDECODEステートメントを使用できます。

  SELECT item_category,
         COUNT (*) total,
         COUNT (DECODE (item_status, 'serviceable', 1)) AS serviceable,
         COUNT (DECODE (item_status, 'under_repair', 1)) AS under_repair,
         COUNT (DECODE (item_status, 'condemned', 1)) AS condemned
    FROM mytable
GROUP BY item_category;

出力：

ITEM_CATEGORY   TOTAL   SERVICEABLE UNDER_REPAIR    CONDEMNED
----------------------------------------------------------------
chair           5       1           2               2
table           5       3           1               1