code: func builtinSqrt(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	if args[0].IsNull() {
  		return args[0], nil
  	}
  
  	sc := ctx.GetSessionVars().StmtCtx
  	f, err := args[0].ToFloat64(sc)
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  
  	// negative value does not have any square root in rational number
  	// Need return null directly.
  	if f < 0 {
  		d.SetNull()
  		return d, nil
  	}
  
  	d.SetFloat64(math.Sqrt(f))
  	return
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_sqrt