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In the previous article we covered queries about order by sorting query results. Next we'll cover some of the other operations.

10. limit Limit the number of query results

In mysql database we want to display the first 10 lines, or line x to line n format display, then limit will be our best choice.

select * from user limit 5; #Show first 5 lines 1 kailinux javadocker2 LInuxmysql NULL3 python NULL4 LInux redis5 linux 0 If we want to see the last three lines, but limit does not support reverse order, we can implement select in the previous way * from user limit 21,3;26 redispyhon 27 PYthon234 28 winner1234 Start with 3 and display 5 select items * from user limit 3,5;4 LInux redis5 linux 06 redis name 7 kailinux javadocker8 LInux redis

11, mysql common functions

concat()

Syntax: CONCAT(str1, str2,...) Can be used to concatenate query results,

select CONCAT(username,'****',password) from user #Concatenate the username and password columns of the user table with 4 ***. Results: kailinux****javadockerLInuxmysql****NULLpython***NULLLInux***redislinux***0 Underline or dash kailinux_javadockerLInuxmysql_NULLpython_NULLLInux_rediskailinux-javadockerLInuxmysql-NULLpython-NULLselect CONCAT(username,'',password) from user #concatenates two columns kailinuxjavadockerLInuxmysqlNULLpythonNULLLInuxredis directly without specifying an interval. From this, we can see that the concat() function can specify any connector when concatenating, so as to display different results of data query.

Random function rand()

This random function can be used to sort randomly in the query results.

select * from user order by rand();5 linux 08 LInux redis15 python javadocker16 LInux 2 LInuxmysql NULL7 kailinux javadocker28 winner1234 20 centos 17 linux 9996 redis name 3 python NULL26 redispyhon 22 rediswinner 25 PYTHoREDIS 27 PYthon234 This kind of random number can be used for lottery and the like, we will randomly sort and get 3 more lines, select * from user order by rand() limit 3;23 PYthonmysql 27 PYthon234 4 LInux redis

count()

select count(*) from user can query how many pieces of data are contained in a table, etc. When selecting columns, generally select columns related to the primary key.

select count(*) from user ;24select count(id ) from user ; but this is slower when the data is large

sum function sum()

select sum(id) from user ; #sum id 356

Find the maximum value max()

select max(id) from user ;28

Find the minimum function ()

select min(id) from user ;1

Average function avg()

select avg(id) from user ;14.8333

group function group by

select username,count(*) from user GROUP BY username ; #Group by username, You can count the information centos of the same user 1kailinux 3LInux 5LInuxmysql 1python 2PYthon234 1PYTHonmysql 2PYTHoREDIS 1redis 3redispyhon 1rediswinner 1ubuntu 1winner1234 1winnerredis 1

For more information on multi-table queries, see the previous article on mysql.

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