數據庫查詢練習-簡單查詢練習及筆記(2)

數據庫查詢練習-簡單查詢練習及筆記(2)

1 分組計算平均成績

查詢每門課的平均成績;

select avg(degree) from score where cno='3-105';
#select avg(要求平均分的字段名稱) from 表名 where 根據那個字段求值='字段值';
/*
mysql> select avg(degree) from score where cno='3-105';
+-------------+
| avg(degree) |
+-------------+
| 85.3333     |
+-------------+
1 row in set (0.02 sec)
*/

如果數據量大 ,一次一次寫比較麻煩,所以用到分組;

select  avg  (degree) from score group by cno;
#select  avg  (要求平均分的字段名稱) from 表名 group by 根據那個字段來求;
/*
mysql> select  avg  (degree) from score group by cno;
+---------------+
| avg  (degree) |
+---------------+
| 85.3333       |
| 76.3333       |
| 81.6667       |
+---------------+
3 rows in set (0.02 sec)

mysql> select cno, avg(degree) from score group by cno;
+-------+-------------+
| cno   | avg(degree) |
+-------+-------------+
| 3-105 | 85.3333     |
| 3-245 | 76.3333     |
| 6-166 | 81.6667     |
+-------+-------------+
3 rows in set (0.02 sec)
*/

2 分組條件與模糊查詢

查詢 score 表中至少有 2 名學生選修，並以 3 開頭的課程的平均分數。

select cno from score group by cno having count(*)>=2 and  cno like '3%';
#select 要查詢的字段 from 在那張表中查詢 group by 以哪個字段來分組 having count(*)>=條件 and  要模糊查詢的字段 like '模糊查詢的字符%';
#    %表示模糊查詢通配符;
/*
mysql> select cno from score group by cno having count(*)>=2 and  cno like '3%';
+-------+
| cno   |
+-------+
| 3-105 |
| 3-245 |
+-------+
2 rows in set (0.03 sec)
*/

3 模糊查詢

通配符:

_表示任意一個字母 定長

%表示多個字母 不定長

select * from  student where sbirthday like  '1974%';
#select 那一個字段*表示所有 from  查那個表 where 模糊查詢的字段 like  '模糊查詢的值%';
/*
mysql> select * from  student where sbirthday like  '1974%';
+-----+--------+------+---------------------+--------+
| sno | sname  | ssex | sbirthday           | sclass |
+-----+--------+------+---------------------+--------+
| 106 | 陸軍   | 男   | 1974-06-03 00:00:00 | 95031  |
| 109 | 趙鐵柱 | 男   | 1974-06-03 00:00:00 | 95031  |
+-----+--------+------+---------------------+--------+
2 rows in set (0.04 sec)
*/