Inquire:
1. Test center: check the scores of account s_01 in all subjects
select id,pin from student where pin='s_01'
select sid,gid,fenshu from student_group where sid=222
select id,name from group where id=333
The account must exist, and the account-subject relationship exists, then the score must exist. If the account-group relationship does not exist, it does not need to be displayed, then:
According to the left join feature, all columns of the left table will be listed. The sql statement is:
selct XX from A left join B on A.XX=B.XX where XXX;
select s.pin as 学生,g.name as 学科,sg.fenshu as 分数 from student as s left join student_group as sg on s.id=sg.sid left join group as g on sg.gid=g.id where s.pin='s_01'
2. Query grouped data, such as querying the total score of all students, test sites: sum function, max, min, avg, group by
select s.pin as 学生,sum(sg.fenshu) as 分数 from student as s left join student_group as sg on s.id=sg.sid where s.pin in('s_01','s_02') group by s.pin
sum() is generally used with group by.
3. select count (distinct A) from XXX, when calculating the total, if there are duplicates, use this syntax to remove duplicates from A.
4. Select distinct A from XXX. When querying results, if there are duplicate items, use this syntax to remove duplicate items from A.
5. Having syntax:
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
order by XXX
eg:
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
HAVING SUM(OrderPrice)<2000
order by Customer desc
Added:
INSERT INTO table name VALUES (value1, value2,....)
eg: INSERT INTO Persons VALUES ('Gates', 'Bill', 'Xuanwumen 10', 'Beijing') (Persons table has 4 columns)
We can also specify the column to insert data into:
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)
eg：INSERT INTO Persons (LastName, Address) VALUES ('Wilson', 'Champs-Elysees')
Revise:
UPDATE table name SET column name = new value WHERE column name = some value
delete:
DELETE FROM table name WHERE column name = value