Mysql Update with if statement

In MySQL

Table1 columns Id, Foo, Bar, Choc, Blk, AB

Table2 columns ID, A, B, C 

Im trying to update table 1 col AB with result of

IF( (C / ((A + B)/ 2))>1, "A...", "B..." ) from Table2 

Where T1.i开发者_开发知识库d =t2.id

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UPDATE table1
JOIN Table2 on Table1.id = Table2.id
SET AB = CASE WHEN C / ((A + B)/ 2) > 1 THEN "A..." ELSE "B..." END