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Database basics exercise part 2
2022-07-06 06:45:00 【Lu__ xiao】
MYSQL(MariaDB) Advanced operation
- order by Usage of
select * from result order by score desc;
take result Data installation scores in the table (score) Sort high and low
among ,desc Representation of descending order ( Decline ); If from low to high ( Ascending ) Arrange , Then you can put desc Switch to asc; If you don't add this parameter , By default, they are arranged in ascending order
select id,name,score from result order by 1;
take result The data in the table is displayed by id Sort
select id,name,score from result order by 2;
take result The data in the table is displayed by name Sort
select id,name,score from result order by 3;
take result The data in the table is displayed by score Sort
select id,name,score from result order by 4;
Prompt error No fourth column
order by Back number (M) Must be less than n( Number of fields in database query ) To display properly . If M>N, The database will report an error
- Limit Usage of
Limit M,N // Says from the first M+1 Data began to look down N Data
Limit M // Indicates before query M Data
select * from result limit 0,2;
The first... In the query table 2 Data
select id,name,score from result limit 1,3;
From 2 Data starts , Go down to query 3 Of data id、name and score Field
- union select Usage of
(1)select * from result union select 1,2,3,4;
The query result of this statement , That is select * from result and union select 1,2,3,4 Splicing of query results
(2) Try the following 3 statement :
select id,name,score from result union select 1,2,3;
Normal display
select id,name,score from result union select 1,2;
Report errors
select id,name,score from result union select 1,2,3,4;
Report errors
The above results are summarized , For the following commands :
Select c1,c2…,cn from result union select d1,d2,…dm;
The second half of the sentence union select Number of fields queried (M) Must be the same as the first half of the sentence select Number of fields queried (n) equal , The database can display the results normally . And order by be similar , This feature can be used to judge the number of fields queried in the database .
(3) Try the following statement
select id,city from result where id=1 and 1=2 union select name,score from result;
From the above results, we can sum up , When the field name is known , An attacker simply places the field anywhere it can be displayed , You can expose the value of this field 、
- union select combination information_schema database
MySQL (MariaDB) 5.5 The above version comes with information schema database , It's about MySQL Information about all other databases maintained by the server , Such as database name, database table 、 Data type and access right of table column . You can put information schema Database as MySQL (MariaDB) Of ” Catalog "!
- Try to execute the following two statements
Show databases;
Select schema_name from information_schema.schemata;
The execution results of the two statements are the same
(2) Try to execute the following two sets of statements
The first group
Use student;
The second group
select table_name from information_schema.tables where table_schema='student';
The execution results of the two groups of commands are the same
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