我可以将查询指定为字段的默认值吗？

Question

I mean something like:

create table Measures (
id_user int,
date timestamp,
measure_1 double default 'select measure_1 from Measures where data = '**/**/****'',
measure_2 double default 'select measure_1 from Measures where data = '**/**/****'');

In this way I insert the value of the last measure saved in the db.. Is it possible?

Answer 1

Not directly:

11.7 Data Type Default Values

... the default value must be a constant; it cannot be a function or an expression.

You'll have to do this on application level, or in a trigger as suggested by @Timekiller.

Answer 2

You can do that via a before-insert trigger .

Check if NEW.measure_1 is null, and if it is, then perform select and store results.

UPD:

Right, I was in a bit of a hurry yesterday, and forgot to give an example later. Trigger is a good replacement for complex default value - it will work transparently, will look just like the default value from database user standpoint, and you won't have to do anything on the application level, since triggers are stored in the database itself. It will look something like this:

CREATE TRIGGER `measures_bi_trigger` BEFORE INSERT ON `Measures`
 FOR EACH ROW BEGIN
if NEW.measure_1 is null then
  SET NEW.measure_1 = (select measure_1 from Measures where ... limit 1);
end if;
if NEW.measure_2 is null then
  SET NEW.measure_2 = (select measure_2 from Measures where ... limit 1);
end if;
END

It's not exactly clear what should be in your where condition, so you'll have to substitute ... yourself. Note that your query should return exactly one row, so either use an aggregate function like MAX or order by ... limit 1 . If your query returns no rows, NULL will be inserted.