MySql将值从一行复制到另一行

Question

This should be pretty basic but i can't figure it out. Have a table 'values'

entity|attribute|value

frank - shirt - red 
frank - hat   - green
sam   - shirt - blue
sam   - hat   - orange

how do i set all hats color's to be the same as the shirt color, of the same person to give frank a red hat and sam a blue hat.

Answer 1

i was looking for a solution to this and i came up on this post. however running the sql lead to a syntax error.

Refering to the mysql docs http://dev.mysql.com/doc/refman/5.0/en/update.html i noticed - at least in version 5 - mysql has a different syntax for this.

the general statement would be:

update table t1, table t2 
set t1.field1 = t2.field2, ..., t1.fieldN = t2.fieldN
where t1.someid = t2.someid and t1.fieldX = '...' and t2.fieldY = '...'

Answer 2

In fact you can not use a subselect from the same table you want to run the update so the best way is to use a join.

UPDATE users AS target
LEFT JOIN users AS source ON source.id = 1
SET target.name = source.name
WHERE target.id = 2;

Answer 3

subselects may be...

set hat color of one random shirt of some person

update values set value = (select value from values where entity = v.entity and attribute = 'shirt' limit 1) from values v where v.attribute = 'hat'

Wrote in browser, so not tested, but you can view an idea.

Answer 4

Give this a try:

UPDATE table 
SET value = res.value
FROM table INNER JOIN table res
ON table.entity = res.entity
WHERE (table.attribute = 'hat') AND (res.attribute = 'shirt')

This is assuming an entity can have at most 1 shirt.