【筆記六】：分組查詢

1，分組查詢

select <列名>, count(*) from <表名> group by <列名>;

查詢每個班級的人數 

MariaDB [mydb]> select class, count(*) from score group by class;
+-------+----------+
| class | count(*) |
+-------+----------+
|     1 |        4 |
|     2 |        1 |
|     3 |        2 |
+-------+----------+
3 rows in set (0.001 sec)

查詢人數大於2的班級

MariaDB [mydb]> select class, count(*) from score group by class having count(*)>2;
+-------+----------+
| class | count(*) |
+-------+----------+
|     1 |        4 |
+-------+----------+
1 row in set (0.002 sec)

查詢數學大於80的各班人數

MariaDB [mydb]> select class, count(*) from score where math > 80 group by class;
+-------+----------+
| class | count(*) |
+-------+----------+
|     1 |        3 |
|     3 |        1 |
+-------+----------+

查詢數學大於80並且人數大於1的班級

MariaDB [mydb]> select class, count(*) from score where math > 80 group by class having count(*)>1;
+-------+----------+
| class | count(*) |
+-------+----------+
|     1 |        3 |
+-------+----------+
1 row in set (0.002 sec)

查詢每個班下的男生和女生人數

MariaDB [mydb]> select * from score;
+-------+-------+-----------+------+------+----------+--------+
| id    | class | name      | sex  | math | language | sports |
+-------+-------+-----------+------+------+----------+--------+
| 20001 |     1 | 小明      | 男   |   81 |       86 |     93 |
| 20002 |     3 | 小紅      | 女   |   86 |       86 |     89 |
| 20003 |     2 | 小張      | 男   |   77 |       83 |     93 |
| 20004 |     1 | 露絲      | 女   |   88 |       78 |     65 |
| 20005 |     1 | 麗麗      | 女   |   92 |       94 |     64 |
| 20006 |     3 | 李明      | 男   |   75 |       78 |     88 |
| 20007 |     1 | 張大明    | 男   |   54 |       65 |     95 |
+-------+-------+-----------+------+------+----------+--------+
7 rows in set (0.000 sec)

MariaDB [mydb]> select class, sex, count(*) from score group by class,sex; 
+-------+------+----------+
| class | sex  | count(*) |
+-------+------+----------+
|     1 | 女   |        2 |
|     1 | 男   |        2 |
|     2 | 男   |        1 |
|     3 | 女   |        1 |
|     3 | 男   |        1 |
+-------+------+----------+
5 rows in set (0.001 sec

MariaDB [mydb]> select class, sex, count(*) from score group by class,sex having count(*) > 1; 
+-------+------+----------+
| class | sex  | count(*) |
+-------+------+----------+
|     1 | 女   |        2 |
|     1 | 男   |        2 |
+-------+------+----------+
2 rows in set (0.002 sec)

2，分頁顯示 

select * from <表名> limit <起始位置>, <顯示條數>;

MariaDB [mydb]> select * from score;
+-------+-------+-----------+------+------+----------+--------+
| id    | class | name      | sex  | math | language | sports |
+-------+-------+-----------+------+------+----------+--------+
| 20001 |     1 | 小明      | 男   |   81 |       86 |     93 |
| 20002 |     3 | 小紅      | 女   |   86 |       86 |     89 |
| 20003 |     2 | 小張      | 男   |   77 |       83 |     93 |
| 20004 |     1 | 露絲      | 女   |   88 |       78 |     65 |
| 20005 |     1 | 麗麗      | 女   |   92 |       94 |     64 |
| 20006 |     3 | 李明      | 男   |   75 |       78 |     88 |
| 20007 |     1 | 張大明    | 男   |   54 |       65 |     95 |
+-------+-------+-----------+------+------+----------+--------+
7 rows in set (0.000 sec)

MariaDB [mydb]> select * from score limit 2,3;
+-------+-------+--------+------+------+----------+--------+
| id    | class | name   | sex  | math | language | sports |
+-------+-------+--------+------+------+----------+--------+
| 20003 |     2 | 小張   | 男   |   77 |       83 |     93 |
| 20004 |     1 | 露絲   | 女   |   88 |       78 |     65 |
| 20005 |     1 | 麗麗   | 女   |   92 |       94 |     64 |
+-------+-------+--------+------+------+----------+--------+
3 rows in set (0.001 sec)

 

select 列名 from 表名
    where  限定條件
        group by 分組的列
            [having  分組之後的篩選條件]
                order by 排序的列  desc/asc
例如：
select studsex,schoolno ,count(*) from studentnew where studteacher ='李老師' GROUP BY schoolno,studsex HAVING count(*)>5;

select * from 表名
    where 篩選條件
        geoup by  --> 分組
            having  -->  分組後的篩選條件
                order by 排序的列  asc/desc
                 limit 1,5開始位置,顯示條數
注：where,order by ,limit 可以單獨出現，也可以組合出現