sg0212148@ubuntu:~$ mysql -uroot
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 40
Server version: 5.1.41-3ubuntu12.10 (Ubuntu)
Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the current input statement.
mysql> create database test;
Query OK, 1 row affected (0.00 sec)
mysql> use test;
Database changed
mysql> create table report (student text, subject text, score int);
Query OK, 0 rows affected (0.00 sec)
insert into report (student, subject, score) values (“st1″,”sb1″,3);
insert into report (student, subject, score) values (“st2″,”sb1″,4);
insert into report (student, subject, score) values (“st3″,”sb1″,5);
insert into report (student, subject, score) values (“st4″,”sb2″,4);
insert into report (student, subject, score) values (“st5″,”sb2″,5);
insert into report (student, subject, score) values (“st6″,”sb2″,6);
insert into report (student, subject, score) values (“st7″,”sb3″,5);
insert into report (student, subject, score) values (“st8″,”sb3″,6);
insert into report (student, subject, score) values (“st9″,”sb3″,7);
mysql> select * from report;
+———+———+——-+
| student | subject | score |
+———+———+——-+
| st1 | sb1 | 3 |
| st2 | sb1 | 4 |
| st3 | sb1 | 5 |
| st4 | sb2 | 4 |
| st5 | sb2 | 5 |
| st6 | sb2 | 6 |
| st7 | sb3 | 5 |
| st8 | sb3 | 6 |
| st9 | sb3 | 7 |
+———+———+——-+
9 rows in set (0.00 sec)
mysql> select student,subject from report where score > 4;
+———+———+
| student | subject |
+———+———+
| st3 | sb1 |
| st5 | sb2 |
| st6 | sb2 |
| st7 | sb3 |
| st8 | sb3 |
| st9 | sb3 |
+———+———+
6 rows in set (0.00 sec)
mysql> select count(subject), subject from (select subject from report where score > 4) as SS group by subject;
+—————-+———+
| count(subject) | subject |
+—————-+———+
| 1 | sb1 |
| 2 | sb2 |
| 3 | sb3 |
+—————-+———+
3 rows in set (0.00 sec)
mysql> select count(subject), subject from (select subject from report where score > 4) as SS group by subject having count(subject) > 1;
+—————-+———+
| count(subject) | subject |
+—————-+———+
| 2 | sb2 |
| 3 | sb3 |
+—————-+———+
2 rows in set (0.00 sec)
‘where‘ operates on individual rows to select row that later may be use for grouping (‘where’ cannot be used after grouping)
‘having‘ operate on agregate functions
you cannot have ‘having’ without group by for proper results:
mysql> select count(subject), subject from (select subject from report where score > 4) as SS having count(subject) > 1;
+—————-+———+
| count(subject) | subject |
+—————-+———+
| 6 | sb1 |
+—————-+———+
