-- 商户分店关系，可重复执行。

SET @has_parent_merchant = (
    SELECT COUNT(*) FROM information_schema.COLUMNS
    WHERE TABLE_SCHEMA = DATABASE()
      AND TABLE_NAME = 'sa_merchant'
      AND COLUMN_NAME = 'parent_merchant_id'
);
SET @add_parent_merchant_sql = IF(
    @has_parent_merchant = 0,
    'ALTER TABLE `sa_merchant` ADD COLUMN `parent_merchant_id` BIGINT UNSIGNED NULL COMMENT ''主商户编号，NULL表示主店'' AFTER `agent_id`, ADD INDEX `idx_merchant_parent` (`parent_merchant_id`)',
    'SELECT 1'
);
PREPARE add_parent_merchant_stmt FROM @add_parent_merchant_sql;
EXECUTE add_parent_merchant_stmt;
DEALLOCATE PREPARE add_parent_merchant_stmt;