mysql fastest queries

[pedroz]

[not sure if this the correct category to post]

although there are some solutions for this...
what is, in your opinion, the fastest queries for this?...

C1 | C2 | C3 | C4
------------------
AA | BB | XX | XX
A1 | B1 | 12 | 34
A2 | B2 | XX | XX

When C3 and C4 = XX, UPDATE C3 = C1 AND C4 = C2

C1 | C2 | C3 | C4
------------------
AA | BB | AA | BB
A1 | B1 | 12 | 34
A2 | B2 | A2 | B2

[r937]

the fastest query is

UPDATE daTable
   SET c3 = c1 
     , c4 = c2
 WHERE c3 = 'XX'
   AND c4 = 'XX'

i know it's the fastest because it's also the only way to do it