column deduplication
1. Use the DISTINCT keyword to deduplicate
SELECT DISTINCT(university)
FROM user_profile
2. Use GROUP BY to group, group back to create a temporary table
SELECT university
FROM user_profile
GROUP BY university
Query results limit the number of rows returned
select device_id from user_profile limit 2;
select device_id from user_profile limit 0,2;
select device_id from user_profile where id <=2;
limit 0, 2 means starting from the first piece of data (0 means the first piece), only 2 pieces of data are displayed per page
The first number indicates the starting address, and the second indicates the offset
Find user information other than Fudan University
select device_id, gender,age,university from user_profile where university!="复旦大学"
Use where to filter empty values exercise
select device_id,gender,age,university from user_profile where age is not null
select device_id,gender,age,university
from user_profile
where age !='
Where in 和Not in
select device_id ,gender,age,university,gpa from user_profile where university in('北京大学','复旦大学','山东大学')