How to fine tune the complex python function for creating merge command to run in bigquery

Question

I created a python code to run the merge command in Google BigQuery, which is used to do UPDATE, INSERT, and DELETE in a single statement. I'd appreciate it if someone could assist me in fine-tuning this code since i believe there are lot of redundant code which can be improved to a greater extent. I have tried my best but cannot make it any more better so need someone's help.

def merge_cmd(database, table, dc, keys, columns, filter_column, new_main_column_type_list):
    try:
        if dc != '-1':
            merge_query = "MERGE INTO " + database + "." + table + "_" + dc + " TARGET USING " + database + "_stg." + table + "_" + dc + "_stg SOURCE "
        else:
            merge_query = "MERGE INTO " + database + "." + table + " TARGET USING " + database + "_stg." + table + "_stg SOURCE "

        if dc != '-1':
            keys = keys + ',dc'
        key_list = keys.split(',')
        col_no_key = columns.split(',')
        for i in range(len(key_list)):
            key_list[i] = key_list[i].lower()

        for i in range(len(col_no_key)):
            col_no_key[i] = col_no_key[i].lower()

        for key in key_list:
            col_no_key.remove(key)
        cols_list = columns.split(',')

        key_list_type = get_key_list_type(key_list, new_main_column_type_list)
        col_no_key_list_type = get_key_list_type(col_no_key, new_main_column_type_list)
        cols_list_type = get_key_list_type(cols_list, new_main_column_type_list)

        ON = " ON "
        b = 1
        for key_type in key_list_type:

            key = key_type.split(':')[0]
            type_ = key_type.split(':')[1]

            ON = ON + " TARGET." + key + " = CAST(SOURCE." + key + " as " + type_ + ")"
            if b < len(key_list):
                ON = ON + " AND "
            b = b + 1

        UPDATE = " WHEN MATCHED THEN UPDATE SET "
        c = 1
        for col_list_type in col_no_key_list_type:

            col = col_list_type.split(':')[0]
            type_ = col_list_type.split(':')[1]

            if col == 'dt':
                UPDATE = UPDATE + " TARGET." + col + " = " + " CAST(SOURCE.calendar_date as date)"
            else:
                UPDATE = UPDATE + " TARGET." + col + " = " + "CAST(SOURCE." + col + " as " + type_ + ")"

            if c < len(col_no_key):
                UPDATE = UPDATE + ","
            c = c + 1

        INSERT = " WHEN NOT MATCHED BY TARGET THEN INSERT (" + columns + ") VALUES ("
        d = 1
        for col_list_type in cols_list_type:

            col = col_list_type.split(':')[0]
            type_ = col_list_type.split(':')[1]

            if col == 'dt':
                INSERT = INSERT + " CAST(SOURCE.calendar_date as date)"
            else:
                INSERT = INSERT + " CAST(SOURCE." + col + " as " + type_ + ")"

            if d < len(cols_list):
                INSERT = INSERT + ","
            d = d + 1
        INSERT = INSERT + ")"

        merge_query = merge_query + ON + UPDATE + INSERT

        merge_query = "'" + merge_query + "'"

        merge_cmd = "bq --location=us query --use_legacy_sql=false " + merge_query

        print(merge_cmd)

        return_cd, out, err = run_sys_command(merge_cmd)

        if return_cd != 0:
            for i in range(20):
                time.sleep(30)
                return_cd, out, err = run_sys_command(merge_cmd)
                if return_cd == 0:
                    break
            if return_cd != 0:
                raise Exception('Merge Failed')