IMPORTANT: Must be run at the *beginning* of a transaction.

    @param in_tables The main input table to select from, followed by a list of

        tables to join with it using the main input table's pkey

    @param mapping dict(out_table_col=in_table_col, ...)

        * out_table_col: str (*not* sql_gen.Col)

        * in_table_col: sql_gen.Col|literal-value

    else: out_pkey = sql.pkey(db, out_table, recover=True)

    out_pkey_col = sql_gen.as_Col(out_pkey, out_table)

    try: in_tables0 = in_tables_.pop(0) # first table is separate

    except IndexError: in_tables0 = None

    else: