Update Query based on condition

Question

I would like to do the following. Update a field based on the value of another field like

update table set if(fielda=1){fieldb=2 fieldc=3}else{fieldd=2 field         


        

Answer 1

I think this syntax will achieve the result you attempted to specify.

UPDATE mytable
   SET fieldb = CASE WHEN fielda = 1 THEN 2 ELSE fieldb END
     , fieldc = CASE WHEN fielda = 1 THEN 3 ELSE fieldc END
     , fieldd = CASE WHEN fielda = 1 THEN fieldd ELSE 2 END
     , fielde = CASE WHEN fielda = 1 THEN fielde ELSE 3 END

The "trick" here is that we are updating all four columns, but in some "cases", we are assigning the current value of the column back to the column, resulting in no real change to the column value. (Once you get your mind bent around that idea, it's pretty easy.)

With MySQL, we do have a handy IF function (not available in most other RDBMS) that we can use to abbreviate that a bit, and achieve the same thing:

UPDATE mytable
   SET fieldb = IF(fielda = 1, 2, fieldb)
     , fieldc = IF(fielda = 1, 3, fieldc)
     , fieldd = IF(fielda = 1, fieldd, 2)
     , fielde = IF(fielda = 1, fielde, 3)

The pain is that you still have to repeat that same conditional test multiple times.

A single scan through the table (like these statements do), and getting all those assignments done in one fell swoop is going to be faster (and more efficient) than breaking this up and doing the assignments piecemeal using multiple statements.