Question from a beginner

[Le pape]

Good morning,
I have table1 and table2.
table1 has 3 columns for exemple (a1,b,c) and table2 has 2 (a2,d) (a is
primary key for both tables)
I create a new column e for table1 wich is b and I want to put all the
values of d in e when a1=a2.

What is the command please ??? I really need that command..

HELP

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Le pape