I don't know if this is what you entended, but if the test fails on one of the if statements none of the following if statements will execute.

Right, but I think my definition of a failed test is different than yours. When the if statement test condition fails or returns false, none of the code inside that if statement is executed so none of the nested if statements following that one will execute. Each one will need to be in its own if-then-end structure and not nested the way you have it.

If you want to check multiple tables, you can't use nested if statements and you can't use else. You need to test each table individually...

let myField1 := 'N° interne';
if cnt(select Aspirateur where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja!)
end;
if cnt(select Bistouri where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja!)
end;
if cnt(select Défibrillateur where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja!)
end

etc...

I would modify the alerts to include the table names so you know which table(s) has/have a match.

let myField1 := 'N° interne';
if cnt(select Aspirateur where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja en table Aspirateur!)
end;
if cnt(select Bistouri where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja en table Bistouri!)
end;
if cnt(select Défibrillateur where 'N° interne' = myField1) > 0 then
alert("Ce N° interne existe déja en table Défibrillateur!)
end