在 Go 中查询 MySQL 的 system variables。跟 SELECT 语句类似，SHOW 请求也返回一个多行结构：

func GetMySQLSystemVariables(db *gorm.DB) error {
	sql := "SHOW VARIABLES WHERE Variable_name IN ('system_time_zone', 'time_zone')"
	var variables []struct {
		VariableName string `gorm:"column:Variable_name"`
		Value        string `gorm:"column:Value"`
	}
	if err := db.Raw(sql).Find(&variables).Error; err != nil {
		return fmt.Errorf("query time zone variables failed: %w", err)
	}
	if len(variables) != 2 {
		return fmt.Errorf("query time zone variables should return 2 rows, got %d", len(variables))
	}

	var systemTZ, tz string
	for _, v := range variables {
		if v.VariableName == "system_time_zone" {
			systemTZ = v.Value
		}
		if v.VariableName == "time_zone" {
			tz = v.Value
		}
	}
	if systemTZ == "" || tz == "" {
		return fmt.Errorf("value of system_time_zone (%s) or time_zone(%s) is empty", systemTZ, tz)
	}
	return nil
}