SQL摘錄筆記 --分組數據（5）

GROUP BY 分組子句

SELECT p_id,COUNT(*) AS n_p
FROM p
GROUP BY p_id

計算每一個id的數目

HAVING過濾分組

SELECT p_id,COUNT(*) AS n_p
FROM p
GROUP BY p_id
HAVING COUNT(*)>=2

也可使用WHERE過濾

 

ORDER BY：對輸出進行排序

GROUP BY：對行分組，單輸出可能不是分組的順序