Mysql method of converting null data: use the IFNULL function. If expr1 is not NULL, [IFNULL()] returns expr1, otherwise it returns expr2, and the code is [IFNULL(expr1, expr2)].
The operating environment of this tutorial: Windows 7 system, mysql version 8.0.22, DELL G3 computer. This method is suitable for all brands of computers.
Related learning recommendations:mysql database
mysql method of converting null data:
Mysql provides the IFNULL function
IFNULL(expr1, expr2)
If expr1 is not NULL, IFNULL() returns expr1, otherwise it returns expr2
Example:
user table structure and data
+----+-----------+ | id | name | +----+-----------+ | 1 | Abby | | 2 | Daisy | | 3 | Christine | +----+-----------+
user_lastlogin table structure and data
+-----+---------------+ | uid | lastlogintime | +-----+---------------+ | 1 | 1488188120 | | 3 | 1488188131 | +-----+---------------+
Query user’s name and lastlogintime
mysql> select a.id,a.name,b.lastlogintime from user as a left join user_lastlogin as b on a.id=b.uid; +----+-----------+---------------+ | id | name | lastlogintime | +----+-----------+---------------+ | 1 | Abby | 1488188120 | | 2 | Daisy | NULL | | 3 | Christine | 1488188131 | +----+-----------+---------------+
Because the user with id=2 has not logged in, there is no record in the user_lastlogin table. So lastlogintime is NULL.
Use IFNULL to convert NULL to 0
IFNULL(lastlogintime, 0)rrree
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