Support \N shortcut for null

[hongyuanw]

For this issue #3685

Support select \N; , allow \N as shortcut for NULL, this sql will return NULL value

Like this ：
tidb> select \N;
+------+
| NULL |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

[zhexuany]

@hanfei1991 @tiancaiamao PTAL.

[shenli]

Why not use s.r.peek()?

[hongyuanw]

@shenli
Before using s.r.peek()，s.r inc() must be called first, because current offset is pointed to '\'.

I want to know if next element is 'N', and don't wanna change offset at the same time .

If next element is not 'N', and s.r.inc() + s.r.peek() are called, we must back to previous offset, it is not convenient .

[winkyao]

simplely add initTokenString("\N", null) in the ruleTable init.

[choleraehyq]

I think you can simply add initTokenString("\N", null) at https://github.com/pingcap/tidb/blob/master/parser/misc.go#L122 .

@tiancaiamao

[winkyao]

LGTM

[choleraehyq]

I think it's ugly to deal with it as a exception here. Would you consider renaming column here https://github.com/pingcap/tidb/blob/master/plan/logical_plan_builder.go#L433 ?

[hongyuanw]

@choleraehyq You are right, doing this work in driver_tidb.go is not good . I have changed it.

[shenli]

Could you add a test case for this?
You can add a test case like select \N; and check the column name and row value.

[choleraehyq]

LGTM
What will happen if a user column is named "\N"?

[tiancaiamao]

Hi, @yuanwhy
You can modify the Scanner.Lex add a special rule:

case null:
    v.item  = nil
    v.ident = "NULL"

and reset changes in plan/logical_plan_builder.go

[hongyuanw]

@choleraehyq
if user input is select "\N", output will show this ：
tidb> select "\N";
+------+
| "\N" |
+------+
| N |
+------+

in mysql , like this :
mysql> select "\N";
+------+
| N |
+------+
| N |
+------+

mysql column name is "\N", but tidb is N . However, it is another issue , select "a" can test this :
tidb> select "a";
+------+
| "a" |
+------+
| a |
+------+

mysql> select "a";
+---+
| a |
+---+
| a |
+---+

I will add test case for select "\N".

[choleraehyq]

@yuanwhy

create table test (`\N` int);
insert into test values (1);
select * from test;

MySQL：
+------+
| \N |
+------+
| 1 |
+------+
will your PR change column name \N to NULL automatically?

[hongyuanw]

1、
@tiancaiamao

case null:
v.item = nil
v.ident = "NULL"

this code in Scanner.Lex doesn't work , because the column name \N is set in parser.go

lastField.SetText(src[lastField.Offset:lastEnd])

That is, parser doesn't use v.ident

2、
@choleraehyq

I have changed code to fix below issue

create table test (\N int);
insert into test values (1);
select * from test;

And some test cases have been added, like :

mysql> select * from test;
+------+
| \N |
+------+
| 1 |
+------+

mysql> select \N from test;
+------+
| NULL |
+------+
| NULL |
+------+

[choleraehyq]

Add a test

select `\N` from test

?
In MySQL this will return \N column.

[choleraehyq]

%s/NUll/NULL

[choleraehyq]

Nice done.
LGTM.