美團(tuán)面試：為什么MySQL不建議使用NULL作為列默認(rèn)值？
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今天來(lái)分享一道美團(tuán)高頻面試題，5 分鐘搞懂“為什么 MySQL 不建議使用 NULL 作為列默認(rèn)值？”。
對(duì)于這個(gè)問(wèn)題，通常能聽(tīng)到的答案是 使用了 NULL 值的列將會(huì)使索引失效,但是如果實(shí)際測(cè)試過(guò)一下,你就知道IS NULL會(huì)使用索引.所以上述說(shuō)法有漏洞.
著急的人拉到最下邊看結(jié)論
前言
Null is a special constraint of columns. The columns in table will be added null constrain if you do not define the column with “not null” key words explicitly when creating the table.Many programmers like to define columns by default because of the conveniences(reducing the judgement code of nullibility) what consequently cause some uncertainty of query and poor performance of database.
NULL值是一種對(duì)列的特殊約束,我們創(chuàng)建一個(gè)新列時(shí),如果沒(méi)有明確的使用關(guān)鍵字not null聲明該數(shù)據(jù)列,Mysql會(huì)默認(rèn)的為我們添加上NULL約束. 有些開(kāi)發(fā)人員在創(chuàng)建數(shù)據(jù)表時(shí),由于懶惰直接使用 Mysql 的默認(rèn)推薦設(shè)置.(即允許字段使用NULL值).而這一陋習(xí)很容易在使用NULL的場(chǎng)景中得出不確定的查詢結(jié)果以及引起數(shù)據(jù)庫(kù)性能的下降.
介紹
Null is null means it is not anything at all,we cannot think of null is equal to ‘’ and they are totally different. MySQL provides three operators to handle null value:“IS NULL”,“IS NOT NULL”,"<=>" and a function ifnull(). IS NULL: It returns true,if the column value is null. IS NOT NULL: It returns true,if the columns value is not null. <=>: It’s a compare operator similar with “=” but not the same.It returns true even for the two null values. (eg. null <=> null is legal) IFNULL(): Specify two input parameters,if the first is null value then returns the second one. It’s similar with Oracle’s NVL() function.
NULL并不意味著什么都沒(méi)有,我們要注意 NULL 跟 ''(空值)是兩個(gè)完全不一樣的值.MySQL 中可以操作NULL值操作符主要有三個(gè).
-
IS NULL -
IS NOT NULL -
<=>太空船操作符,這個(gè)操作符很像=,select NULL<=>NULL可以返回true,但是select NULL=NULL返回false. -
IFNULL一個(gè)函數(shù).怎么使用自己查吧…反正我會(huì)了
Example
Null never returns true when comparing with any other values except null with “<=>”.
NULL通過(guò)任一操作符與其它值比較都會(huì)得到NULL,除了<=>.
(root@localhost mysql3306.sock)[zlm]>create table test_null(
-> id int not null,
-> name varchar(10)
-> );
Query OK, 0 rows affected (0.02 sec)
(root@localhost mysql3306.sock)[zlm]>insert into test_null values(1,'zlm');
Query OK, 1 row affected (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>insert into test_null values(2,null);
Query OK, 1 row affected (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null;
+----+------+
| id | name |
+----+------+
| 1 | zlm |
| 2 | NULL |
+----+------+
2 rows in set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where name=null;
Empty set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is null;
+----+------+
| id | name |
+----+------+
| 2 | NULL |
+----+------+
1 row in set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is not null;
+----+------+
| id | name |
+----+------+
| 1 | zlm |
+----+------+
1 row in set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where null=null;
Empty set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<>null;
Empty set (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<=>null;
+----+------+
| id | name |
+----+------+
| 1 | zlm |
| 2 | NULL |
+----+------+
2 rows in set (0.00 sec)
//null<=>null always return true,it's equal to "where 1=1".
Null means “a missing and unknown value”.Let’s see details below.
NULL 代表一個(gè)不確定的值,就算是兩個(gè) NULL,它倆也不一定相等.(像不像 C 中未初始化的局部變量)
(root@localhost mysql3306.sock)[zlm]>SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL;
+-----------+---------------+------------+----------------+
| 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL |
+-----------+---------------+------------+----------------+
| 0 | 1 | 0 | 1 |
+-----------+---------------+------------+----------------+
1 row in set (0.00 sec)
//It's not equal to zero number or vacant string.
//In MySQL,0 means fasle,1 means true.
(root@localhost mysql3306.sock)[zlm]>SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
| NULL | NULL | NULL | NULL |
+----------+-----------+----------+----------+
1 row in set (0.00 sec)
//It cannot be compared with number.
//In MySQL,null means false,too.
It truns null as a result if any expression contains null value.
任何有返回值的表達(dá)式中有NULL參與時(shí),都會(huì)得到另外一個(gè)NULL值.
(root@localhost mysql3306.sock)[zlm]>select ifnull(null,'First is null'),ifnull(null+10,'First is null'),ifnull(concat('abc',null),'First is null');
+------------------------------+---------------------------------+--------------------------------------------+
| ifnull(null,'First is null') | ifnull(null+10,'First is null') | ifnull(concat('abc',null),'First is null') |
+------------------------------+---------------------------------+--------------------------------------------+
| First is null | First is null | First is null |
+------------------------------+---------------------------------+--------------------------------------------+
1 row in set (0.00 sec)
//null value needs to be disposed with ifnull() function,what usually causes sql statement more complex.
//As we all know,MySQL does not support funcion index.Therefore,indexes on the column may not be used.That's really worse.
It’s diffrent when using count(*) & count(null column).
使用count(*) 或者 count(null column)結(jié)果不同,count(null column)<=count(*).
(root@localhost mysql3306.sock)[zlm]>select count(*),count(name) from test_null;
+----------+-------------+
| count(*) | count(name) |
+----------+-------------+
| 2 | 1 |
+----------+-------------+
1 row in set (0.00 sec)
//count(*) returns all rows ignore the null while count(name) returns the non-null rows in column "name".
//This will also leads to uncertainty if someone is unaware of the details above.
When using distinct,group by,order by,all null values are considered as the same value.
雖然select NULL=NULL的結(jié)果為false,但是在我們使用distinct,group by,order by時(shí),NULL又被認(rèn)為是相同值.
(root@localhost mysql3306.sock)[zlm]>insert into test_null values(3,null);
Query OK, 1 row affected (0.00 sec)
(root@localhost mysql3306.sock)[zlm]>select distinct name from test_null;
+------+
| name |
+------+
| zlm |
| NULL |
+------+
2 rows in set (0.00 sec)
//Two rows of null value returned one and the result became two.
(root@localhost mysql3306.sock)[zlm]>select name from test_null group by name;
+------+
| name |
+------+
| NULL |
| zlm |
+------+
2 rows in set (0.00 sec)
//Two rows of null value were put into the same group.
//By default,group by will also sort the result(null row showed first).
(root@localhost mysql3306.sock)[zlm]>select id,name from test_null order by name;
+----+------+
| id | name |
+----+------+
| 2 | NULL |
| 3 | NULL |
| 1 | zlm |
+----+------+
3 rows in set (0.00 sec)
//Three rows were sorted(two null rows showed first).
MySQL supports to use index on column which contains null value(what’s different from oracle).
MySQL 中支持在含有NULL值的列上使用索引,但是Oracle不支持.這就是我們平時(shí)所說(shuō)的如果列上含有NULL那么將會(huì)使索引失效.
嚴(yán)格來(lái)說(shuō),這句話對(duì)與 MySQL 來(lái)說(shuō)是不準(zhǔn)確的.
(root@localhost mysql3306.sock)[sysbench]>show tables;
+--------------------+
| Tables_in_sysbench |
+--------------------+
| sbtest1 |
| sbtest10 |
| sbtest2 |
| sbtest3 |
| sbtest4 |
| sbtest5 |
| sbtest6 |
| sbtest7 |
| sbtest8 |
| sbtest9 |
+--------------------+
10 rows in set (0.00 sec)
(root@localhost mysql3306.sock)[sysbench]>show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`k` int(11) NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=100001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
(root@localhost mysql3306.sock)[sysbench]>alter table sbtest1 modify k int null,modify c char(120) null,modify pad char(60) null;
Query OK, 0 rows affected (4.14 sec)
Records: 0 Duplicates: 0 Warnings: 0
(root@localhost mysql3306.sock)[sysbench]>insert into sbtest1 values(100001,null,null,null);
Query OK, 1 row affected (0.00 sec)
(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where id=100001;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | sbtest1 | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where k is null;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
| 1 | SIMPLE | sbtest1 | NULL | ref | k_1 | k_1 | 5 | const | 1 | 100.00 | Using where; Using index |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
//In the first query,the newly added row is retrieved by primary key.
//In the second query,the newly added row is retrieved by secondary key "k_1"
//It has been proved that indexes can be used on the columns which contain null value.
//column "k" is int datatype which occupies 4 bytes,but the value of "key_len" turn out to be 5.what's happed?Because null value needs 1 byte to store the null flag in the rows.
這個(gè)是我自己測(cè)試的例子.
mysql> select * from test_1;
+-----------+------+------+
| name | code | id |
+-----------+------+------+
| gaoyi | wo | 1 |
| gaoyi | w | 2 |
| chuzhong | wo | 3 |
| chuzhong | w | 4 |
| xiaoxue | dd | 5 |
| xiaoxue | dfdf | 6 |
| sujianhui | su | 99 |
| sujianhui | NULL | 99 |
+-----------+------+------+
8 rows in set (0.00 sec)
mysql> explain select * from test_1 where code is NULL;
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | test_1 | NULL | ref | index_code | index_code | 161 | const | 1 | 100.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from test_1 where code is not NULL;
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | test_1 | NULL | range | index_code | index_code | 161 | NULL | 7 | 100.00 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from test_1 where code='dd';
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | test_1 | NULL | ref | index_code | index_code | 161 | const | 1 | 100.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from test_1 where code like "dd%";
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | test_1 | NULL | range | index_code | index_code | 161 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
總結(jié)
null value always leads to many uncertainties when disposing sql statement.It may cause bad performance accidentally.
列中使用NULL值容易引發(fā)不受控制的事情發(fā)生,有時(shí)候還會(huì)嚴(yán)重托慢系統(tǒng)的性能.
例如:
null value will not be estimated in aggregate function() which may cause inaccurate results.
對(duì)含有 NULL 值的列進(jìn)行統(tǒng)計(jì)計(jì)算,eg. count(),max(),min(),結(jié)果并不符合我們的期望值.
null value will influence the behavior of the operations such as “distinct”,“group by”,“order by” which causes wrong sort.
干擾排序，分組,去重結(jié)果.
null value needs ifnull() function to do judgement which makes the program code more complex.
有的時(shí)候?yàn)榱讼?code style="font-size: 14px;padding: 2px 4px;border-radius: 4px;margin-right: 2px;margin-left: 2px;background-color: rgba(27, 31, 35, 0.05);font-family: 'Operator Mono', Consolas, Monaco, Menlo, monospace;word-break: break-all;color: rgb(41, 128, 185);">NULL帶來(lái)的技術(shù)債務(wù),我們需要在 SQL 中使用IFNULL()來(lái)確保結(jié)果可控,但是這使程序變得復(fù)雜.
null value needs a extra 1 byte to store the null information in the rows.
NULL值并是占用原有的字段空間存儲(chǔ),而是額外申請(qǐng)一個(gè)字節(jié)去標(biāo)注,這個(gè)字段添加了NULL約束.(就像額外的標(biāo)志位一樣)
As these above drawbacks,it’s not recommended to define columns with default null. We recommand to define “not null” on all columns and use zero number & vacant string to substitute relevant data type of null.
根據(jù)以上缺點(diǎn),我們并不推薦在列中設(shè)置 NULL 作為列的默認(rèn)值,你可以使用NOT NULL消除默認(rèn)設(shè)置,使用0或者''空字符串來(lái)代替NULL。
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