MySQL：将新列插入到具有其他表默认值的表中

Question

I want to add a new column to one table with default value from other table's column.

1st Table 'sensors'
-------------------------------------------------------------
id    sensorId      location    city    country     userId     
-------------------------------------------------------------

2nd Table sensordata
-------------------------------
id    sensorId     dataValues  
-------------------------------

I want to add a column in the sensordata table with the default value of location where sensorId is same.

How can I do that?

Something like this

ALTER TABLE  `sensordata` s ADD  `location` VARCHAR( 200 ) NOT NULL DEFAULT 
    (SELECT location from sensors s WHERE s.sensorId = d.sensorId) AFTER  `dataValues`

Please don't tell me why I need to have duplicate data, there is a reason for that :)

Answer 1

You need to do it in 2 steps

step1

ALTER TABLE  `sensordata`  ADD  `location` VARCHAR( 200 )  ;

step2

update sensordata sd
join sensors s on s.sensorId = sd.sensorId
set sd.location = s.location

Updated: This is what I have done and it works fine

ALTER TABLE `sensordata` ADD `location` VARCHAR(200) NULL DEFAULT NULL AFTER `dataValues`;

UPDATE sensordata d SET  d.location = (SELECT s.location from sensors s WHERE s.sensorID = d.sensorID);

Answer 2

Mysql requires default value to be a constant in almost all cases - https://dev.mysql.com/doc/refman/5.0/en/data-type-defaults.html . You can write before insert trigger though to emulate DEFAULT functionality to some extent. Surely, trigger will affect only new rows, and you will need to manually update existing.