The method of converting null data by mysql
Editor to share with you the method of mysql conversion null data, I believe that most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!
How mysql converts null data: use the IFNULL function. If expr1 is not NULL, [IFNULL ()] returns expr1, otherwise it returns expr2 with the code [IFNULL (expr1, expr2)].
How mysql converts null data:
Mysql provides the IFNULL function
IFNULL (expr1, expr2)
Returns expr1 if expr1 is not NULL,IFNULL (), otherwise returns expr2
Example:
User structure and data
+-+-+ | id | name | +-+-+ | 1 | Abby | | 2 | Daisy | | 3 | Christine | +-+-+
User_ lastlogin table structure and data
+-+ | uid | lastlogintime | +-+-+ | 1 | 1488188120 | | 3 | 1488188131 | +-+-+
Query name and lastlogintime of user
Mysql > select a. ID. A. Name. B. Lastlogintime from user as a left join user_lastlogin as b on a.id=b.uid +-1 | Abby | 1488188120 | 2 | Daisy | NULL | 3 | Christine | 1488188131 | +-+ -+
Because the user of id=2 has not logged in, it is not recorded in the user_lastlogin table. So lastlogintime is NULL.
Use IFNULL to convert NULL to 0
IFNULL (lastlogintime,0) mysql > select a.id as lastlogintime from user as a left join user_lastlogin as b on a.id=b.uid a.name as lastlogintime from user as a left join user_lastlogin as b on a.id=b.uid if null (b.lastlogintimepenol 0) +-1 | Abby | 1488188120 | 2 | Daisy | 0 | 3 | Christine | 1488188131 | +- -+-+ these are all the contents of the article "how mysql transforms null data" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!
