sql base (1)

1, where, and having keywords

where and having filter conditions can be used in the sql statement, group by statement and can be used together.

Two differences:

(1) when used with group by, where before the group by statement, where the representative first filtered with a statement, and then use the results of the group by filtration packets;

And having statements can only be used after the group by, the result grouped for screening.

After the condition (2) where the expression is not allowed to use a polymerization function , when the need to use a polymerization function can only be on the back having.

Exercise sql: query average score less than 60 points and the number of students in the student's name and student grade point average - (including performance and non-performance)

--学生表
CREATE TABLE `Student`(
`s_id` VARCHAR(20),
`s_name` VARCHAR(20) NOT NULL DEFAULT '',
`s_birth` VARCHAR(20) NOT NULL DEFAULT '',
`s_sex` VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(`s_id`)
)
--成绩表
CREATE TABLE `Score`(
    `s_id` VARCHAR(20),
    `c_id`  VARCHAR(20),
    `s_score` INT(3),
    PRIMARY KEY(`s_id`,`c_id`)
);

The SELECT a.s_id student numbers, ROUND ( AVG (a.s_score), 2 ) grade point average, b.s_name student name   the FROM Score A
 LEFT  JOIN Student b ON a.`s_id` = b.`s_id`
 the GROUP  BY a.s_id , b.s_name
 the HAVING  AVG (a.s_score) <  60  
UNION 
the SELECT a.s_id student number, 0 grade point average, a.s_name student name   the FROM student A
 the WHERE a.`s_id` the NOT  the IN ( the SELECT  DISTINCT score.s_id the FROM score)

Guess you like