mysql - Auto increment null fields, leave non null fields as is -

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i have table items has field priority. half of elements in field priority have unique value ranging 1 x. want elements field priority null have incremental values of x+1 y.

i have tried

update items set priority = ifnull(priority, 0)

but not want. want non null fields stay is, achieved following:

set priority = ifnull(priority, 0)

and null fields start being incremented value, following:

select max(priority) items;

so thing comes mind close to

update items set priority =     ifnull(priority, 0) + (select max(priority)                            items) priority null

however not sure how go doing so. hints or tips?

you can current maximum priority , store in variable. then, bring value query, using cross join. can increment value in set statement:

update items cross join        (select @x := max(priority) items) maxp     set priority = (@x := @x + 1)     priority null;

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