SQL exercise (1) Write an UPDATE statement, calculate the average score of each student and fill it in the avg_grade field in the student table.

Question stem:

Write an UPDATE statement to calculate the average score of each student and fill it in the avg_grade field in the student table.

Table Structure:

The SQL statement defining the table structure is as follows:

CREATE TABLE student (

sno varchar(6) NOT NULL ,

sname varchar(10) ,

sex char(2) ,

nation char(2) ,

pnum char(18) ,

birth date ,

phone char( 11) ,

dept varchar(20) ,

avg_grade float(4,2),

PRIMARY KEY (sno)

) ;

CREATE TABLE score (

sno varchar(6) NOT NULL,

cno varchar(6) NOT NULL,

term varchar(15),

grade int(11),

PRIMARY KEY (sno,cno)

) ;

table sample

The table sample corresponding to the above table structure, some data are as follows:

studentsurface:

scoresurface:

Sample output:

Sample output, part of the data is as follows:

When writing this question, there was an error that I have never encountered before:

mysql报错：1093 - You can't specify target table 'student' for update in FROM clause

It means: In a SQL statement, you cannot first query a certain value in a table and then change the table

It is not possible to query and modify in the same table at the same time

My code: directly modify the average value obtained from the query as the modified content

Solution: Use the found information as a new virtual table, perform query work in the virtual table and then modify the student table

ac source code:

update student
set avg_grade=(
select s.avg_grade from(
select student.sno,round(avg(grade),2) as avg_grade
from score,student
where score.sno = student.sno
group by student.sno
)s
where student.sno = s.sno)