My idea is quite simple, make a query and use both an external cell and a column from within the query range as parameters of a function.

As an example, this is the given sheet:

And those were the ways I tried out and their results:

QUERY(A1:B5; "select sum(b) where "&EQ(C1;&"A"&)&"=true"; 0)

This one throws a function error.

QUERY(A1:B5; "select sum(b) where "&EQ(C1;A)&"=true"; 0)

This one says A is an unknown name.

QUERY(A1:B5; "select sum(b) where "&EQ(C1;A1)&"=true"; 0)

This return the sum of all values in column B (150), meaning no rows were filtered.

The expected result was for the query function read all rows and filter any where the value in column A is not equal to C1.

Is something like that achievable?

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Edit: Okay, so there is a solution using ArrayFormula and concatenating it with the Query range:

QUERY({A1:B5,ARRAYFORMULA(EQ(C1,A1:B5))}; "select sum(Col2) where (Col3=true)"; 0)

This should work, but I'll keep it open until we find a solution using Query and EQ as previously intended.