Syntax to find value

Hello Stopira 

you can join both tables and transform the joined table (extracting first row of Category). Another approach could be to add a new column where filtering the other table and extract again first row of Category. Here an example code of the first approach

let
    TATable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtK31DcyMDJQ0lHy9tY1sNQ1MDAwhHLAzFidaCVDAyyqjBCqjMCqjMyxqDJGqDJWio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Transaction Date" = _t, #"Transaction Number" = _t, #"Customer Account" = _t]),
    SATable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8vbWNbDUNTAwMFTSUQooyk8pTS6BcDxLUnMVHBGiQJYCCJgaGOgA1esZGCjF6uAywAjIcUpMB+mB6UMHugoKqAYYIRtgDHOBE6YLjHC4AMUAE5gBzpgGGOMwwBjZAFOYAS5oBhiC9ML1xwIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Transaction Number" = _t, #"Item ID" = _t, #"Item name" = _t, Category = _t, Amount = _t]),
    JoinTables = Table.NestedJoin
    (
        TATable, 
        "Transaction Number",
        SATable, 
        "Transaction Number",
        "SATable"
    ), 
    TransformJoinedTable = Table.TransformColumns
    (
        JoinTables,
        {
            {
                "SATable",
                (tbl)=> try tbl[Category]{0} otherwise null,
                type text
            }
        }
    )

in
    TransformJoinedTable

Copy paste this code to the advanced editor in a new blank query to see how the solution works.

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Have fun

Jimmy