From 1fa56cff16e5855ae4b5bdcecdfa47e96fde112f Mon Sep 17 00:00:00 2001
From: qw4990 <qw4990@163.com>
Date: Wed, 8 May 2019 16:43:56 +0800
Subject: [PATCH] add comments

---
 expression/builtin_time.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/expression/builtin_time.go b/expression/builtin_time.go
index 3f49ca53138d0..883cad23a64de 100644
--- a/expression/builtin_time.go
+++ b/expression/builtin_time.go
@@ -4734,6 +4734,7 @@ func (c *periodAddFunctionClass) getFunction(ctx sessionctx.Context, args []Expr
 	return sig, nil
 }
 
+// validPeriod checks if this period is valid, it comes from MySQL 8.0+.
 func validPeriod(p int64) bool {
 	return !(p < 0 || p%100 == 0 || p%100 > 12)
 }
@@ -4794,6 +4795,7 @@ func (b *builtinPeriodAddSig) evalInt(row chunk.Row) (int64, bool, error) {
 		return 0, true, err
 	}
 
+	// in MySQL, if p is invalid but n is NULL, the result is NULL, so we have to check if n is NULL first.
 	if !validPeriod(p) {
 		return 0, false, errIncorrectArgs.GenWithStackByArgs("period_add")
 	}