Table of contents
1. Query the table content and add one more column
2. Add new column names to the data table
1. Average calculation AVG command
2. Sum command to calculate sum
1. Query the table content and add one more column
Enter select * from mpy; view the table contents
2. Add new column names to the data table
Enter alter table mpy add ip1 int(20); (modify a table name, add means I want to add a new column name, and add attributes after the column name)
Enter select * from mpy again; query
3. Subquery
select * from test where age > (select avg(age) from test);
(When the age is greater than the average value I queried, the content of the age information is retained). These are two SQL statements. However, when comparing operators, they are not one value, but are obtained through SQL statement query. Comes worth it.
4. Aggregation function
1. Average calculation AVG command
select avg(colume1) from test;
Create a data table called classes and view
Enter the select avg(age) from banji; command to calculate the average of age
The average was successfully calculated to be 39.27
2. Sum command to calculate sum
import select sum(age) from banji;
Successfully obtained the sum of the total number 432
3. count command
Used to count the number of rows in a specified column, excluding non-empty rows
select count(age) from banji;
Successfully calculated 11 columns
4. MAX command
Maximum value in specified column
select max(age) from banji;
5. MIN command
Minimum value in specified list
select min(age) from banji;
5. Multi-table query
1. Inner join query
First check the content of the two tables that are incorrectly communicated with each other.
The input content mpy represents the first table, banji represents the second table, and .name represents the column content in the table.
2. External connection query
1. Left outer join query
Enter the following command (data that matches the table on the left is displayed)
mysql> select *
-> from banji
-> left join mpy
-> on banji.name = mpy.name;
2. Right outer join query
Enter the following command (data that matches the table on the right is displayed)
mysql> select *
-> from mpy
-> left join banji
-> on mpy.name = banji.name;
6. Summary
Database content is indispensable for the current Internet environment. Addition, deletion, modification, and multi-table query are very important links. There is a high probability that these questions will be asked in interviews, so you must practice more and stick to them. Have a firm grasp.