where VS having

• WHERE過濾行，而HAVING過濾分組
• WHERE後不能使用聚合函式過濾行，而HAVING可以使用聚合函式過濾分組
• WHERE在資料分組前進行過濾，HAVING在資料分組後進行過濾

分組彙總 WITH ROLLUP

select class,ssex,count(sname) from student group by class,ssex;
+-------+------+--------------+
| class | ssex | count(sname) |
+-------+------+--------------+
| 95031 | 女   |            1 |
| 95031 | 男   |            2 |
| 95033 | 女   |            1 |
| 95033 | 男   |            2 |
+-------+------+--------------+
 

select class,ssex,count(sname) from student group by class,ssex with rollup;
+-------+------+--------------+
| class | ssex | count(sname) |
+-------+------+--------------+
| 95031 | 女   |            1 |
| 95031 | 男   |            2 |
| 95031 | NULL |            3 |
| 95033 | 女   |            1 |
| 95033 | 男   |            2 |
| 95033 | NULL |            3 |
| NULL  | NULL |            6 |
+-------+------+--------------+