sql - update column of table with other column -

i hava table a

col1    col2 ------------- 1         2         hhhh 3         erer 4         sdfsdfds 5          alimd

table has relationshib other tables.

and other table name b

col1     col2 ---------------- 1        hhjgjh 2         jkkjerwe 3          jjjjj

tables , b have milions of records

question : want update col2 of table col2 of table b

the best , speed of query update

thanks

update set a.col2 = b.col2 tablea inner join tableb b on a.col1 = b.col1

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