【MySQL】 IS NOT NULL 和 != NULL 的区别？

背景

最近在开发小伙伴的需求遇到了一个数据库统计的问题
is not null 结果正确
=null 结果就不对然后就激发了获取真理的想法那必须的查查
咋回事嘞

开整

在用MySQL的过程中你是否存在过如下的几个疑问

• 我的字段类型明明指定的是NOT NULL但是为什么还是可以插入空值呢
• 为什么NOT NULL的效率比NULL更高
• 在查询空字段的记录时是用"select * from table where col <> ‘’ “还是用"select * from table where col is not null”

带着疑问我们来看看NOT NULL和NULL有什么不一样呢要搞清楚这两个的区别。首先我们先要理解"空值"和"NULL"的含义

• 空值是不占用空间的。
• NULL是会占用空间的我们来看看官方对这个NULL的描述。MySQL的官方描述如下

“NULL columns require additional space in the row to record whether their values are NULL. For MyISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte.”

怎么理解呢简单举个栗子

假设有一个瓶子空值表示的是瓶子里什么都没有NULL表示的是瓶子里面状态的是空气。可以理解为什么NULL也会占用空间了吧。

下面来通过一个实例例子来测试一下。首先建一个表表引擎使用InnoDB建表语句如下

create table test(
  c1 varchar(10) not null, 
  c2 varchar(10) default null
) engine = InnoDB;

验证插入数据和查询

mysql> insert into test(c1, c2) values(null, 0);
ERROR 1048 (23000): Column 'c1' cannot be null

mysql> insert into test(c1, c2) values('', 0);
Query OK, 1 row affected (0.00 sec)
# null字符串
mysql> insert into test(c1, c2) values('null', 0);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test(c1, c2) values('', null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+------+------+
| c1   | c2   |
+------+------+
|      | 0    |
| null | 0    |
|      | NULL |
+------+------+
3 rows in set (0.00 sec)

从上述结果中可以看到NOT NULL的字段是没办法插入NULL值的只能插入空值’'。上面第三个SQL插入的NULL是个字符串格式的NULL对于MyISAM的存储引擎测试的结果和上述结果是一样的。所以第1个疑问也就了解了吧。

对于第2个问题因为NULL值是占了一定空间的所以在MySQL进行字段比较的时候值为NULL的字段也是会参与比较的所以是会对性能有一定的影响。

当字段上包含有索引时由于B树索引是不会存储NULL值的所以在使用这个字段做为查询条件时对性能的影响还是比较大的在平时创建索引的时候应该尽量保证列的值不为NULL。

针对上述的结论有几个针对NULL和NOT NULL的常见优化建议

• MySQL如果不指定列的约束默认就是允许NULLTIMESTAMP类型的字段除外。所以在非必要情况下尽量设置列的约束为NOT NULL。
• 如果列的值为NULL通过这个为NULL的列进行条件查询时MySQL更难做优化因为为NULL的列会让索引的统计和值的比较更加复杂。
• 如果计划在某个列上创建索引那么需要尽量避免这个列中的字段值为NULL。在优化的过程中把NULL改为NOT NULL对性能的提升并不是很明显。所以如果在使用过程中没有问题的话没有必要首先去做NULL到NOT NULL的优化。

来通过实际例子看看最后一个问题。假如需要查询上述test表中c1不为空的所有数据应该使用"<> ‘’"呢还是使用"IS NOT NULL"呢测试结果如下

mysql> select * from test where c1 is not null;
+------+------+
| c1   | c2   |
+------+------+
|      | 1    |
| null | 1    |
|      | NULL |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from test where c1 <> '';
+------+------+
| c1   | c2   |
+------+------+
| null | 1    |
+------+------+
1 row in set (0.00 sec)

可以看到不同的查询条件对于查询的结果区别还是特别大的。所以在使用过程中需要根据业务场景选择不同的查询条件。

附录

附录1MySQL索引失效的常见情况

1. 最左前缀原则。例如存在联合索引idx_a_b(a, b)查询条件使用where b = 1则无法使用索引
2. LIKE 前置模糊查询。例如col_name like '%test' 或 col_name like '%test%'
3. 索引列使用函数或存在计算。例如存在索引idx_col(col)查询条件使用where left(col, 2) = 'te'
4. 查询条件使用 is not null。设计表结构时尽量设置 not null约束
5. 字段类型出现隐式转换。例如存在字段test_col为varchar类型查询时使用了where test_col = 1隐式转为了int类型导致索引失效
6. 条件中有 or 存在可能不会使用索引。例如查询条件为where a = 'testa' or b = 'testb'存在索引idx_a(a)此时也不会使用索引除非为b字段也添加索引
7. 查询结果超过整体结果的25%或三分之一或者表数据量比较少时MySQL认为全表扫描代价更小会导致索引失效

附录2MySQL关键字列表

R 表示为 MySQL 预留关键字

关键字	关键字	关键字
ACCESSIBLER	ACCOUNT	ACTION
ADDR	AFTER	AGAINST
AGGREGATE	ALGORITHM	ALLR
ALTERR	ALWAYS	ANALYSE
ANALYZER	ANDR	ANY
ASR	ASCR	ASCII
ASENSITIVER	AT	AUTOEXTEND_SIZE
AUTO_INCREMENT	AVG	AVG_ROW_LENGTH
BACKUP	BEFORER	BEGIN
BETWEENR	BIGINTR	BINARYR
BINLOG	BIT	BLOBR
BLOCK	BOOL	BOOLEAN
BOTHR	BTREE	BYR
BYTE	CACHE	CALL ®
CASCADE ®	CASCADED	CASE ®
CATALOG_NAME	CHAIN	CHANGE ®
CHANGED	CHANNEL	CHAR ®
CHARACTER ®	CHARSET	CHECK ®
CHECKSUM	CIPHER	CLASS_ORIGIN
CLIENT	CLOSE	COALESCE
CODE	COLLATE ®	COLLATION
COLUMN ®	COLUMNS	COLUMN_FORMAT
COLUMN_NAME	COMMENT	COMMIT
COMMITTED	COMPACT	COMPLETION
COMPRESSED	COMPRESSION	CONCURRENT
CONDITION ®	CONNECTION	CONSISTENT
CONSTRAINT ®	CONSTRAINT_CATALOG	CONSTRAINT_NAME
CONSTRAINT_SCHEMA	CONTAINS	CONTEXT
CONTINUE ®	CONVERT ®	CPU
CREATE ®	CROSS ®	CUBE
CURRENT	CURRENT_DATE ®	CURRENT_TIME ®
CURRENT_TIMESTAMP ®	CURRENT_USER ®	CURSOR ®
CURSOR_NAME	DATA	DATABASE ®
DATABASES ®	DATAFILE	DATE
DATETIME	DAY	DAY_HOUR ®
DAY_MICROSECOND ®	DAY_MINUTE ®	DAY_SECOND ®
DEALLOCATE	DEC ®	DECIMAL ®
DECLARE ®	DEFAULT ®	DEFAULT_AUTH
DEFINER	DELAYED ®	DELAY_KEY_WRITE
DELETE ®	DESC ®	DESCRIBE ®
DES_KEY_FILE	DETERMINISTIC ®	DIAGNOSTICS
DIRECTORY	DISABLE	DISCARD
DISK	DISTINCT ®	DISTINCTROW ®
DIV ®	DO	DOUBLE ®
DROP ®	DUAL ®	DUMPFILE
DUPLICATE	DYNAMIC	EACH ®
ELSE ®	ELSEIF ®	ENABLE
ENCLOSED ®	ENCRYPTION	END
ENDS	ENGINE	ENGINES
ENUM	ERROR	ERRORS
ESCAPE	ESCAPED ®	EVENT
EVENTS	EVERY	EXCHANGE
EXECUTE	EXISTS ®	EXIT ®
EXPANSION	EXPIRE	EXPLAIN ®
EXPORT	EXTENDED	EXTENT_SIZE
FALSE ®	FAST	FAULTS
FETCH ®	FIELDS	FILE
FILE_BLOCK_SIZE	FILTER	FIRST
FIXED	FLOATR	FLOAT4R
FLOAT8R	FLUSH	FOLLOWS
FORR	FORCER	FOREIGNR
FORMAT	FOUND	FROMR
FULL	FULLTEXTR	FUNCTION
GENERAL	GENERATEDR	GEOMETRY
GEOMETRYCOLLECTION	GETR	GET_FORMAT
GLOBAL	GRANTR	GRANTS
GROUPR	GROUP_REPLICATION	HANDLER
HASH	HAVINGR	HELP
HIGH_PRIORITYR	HOST	HOSTS
HOUR	HOUR_MICROSECONDR	HOUR_MINUTER
HOUR_SECONDR	IDENTIFIED	IFR
IGNORER	IGNORE_SERVER_IDS	IMPORT
INR	INDEXR	INDEXES
INFILER	INITIAL_SIZE	INNERR
INOUTR	INSENSITIVER	INSERTR
INSERT_METHOD	INSTALL	INSTANCE
INTR	INT1R	INT2R
INT3R	INT4R	INT8R
INTEGERR	INTERVALR	INTOR
INVOKER	IO	IO_AFTER_GTIDSR
IO_BEFORE_GTIDSR	IO_THREAD	IPC
ISR	ISOLATION	ISSUER
ITERATER	JOINR	JSON
KEYR	KEYSR	KEY_BLOCK_SIZE
KILLR	LANGUAGE	LAST
LEADINGR	LEAVE ®	LEAVES
LEFT ®	LESS	LEVEL
LIKE ®	LIMIT ®	LINEARR
LINESR	LINESTRING	LIST
LOADR	LOCAL	LOCALTIMER
LOCALTIMESTAMPR	LOCKR	LOCKS
LOGFILE	LOGS	LONGR
LONGBLOBR	LONGTEXTR	LOOPR
LOW_PRIORITYR	MASTER	MASTER_AUTO_POSITION
MASTER_BINDR	MASTER_CONNECT_RETRY	MASTER_DELAY
MASTER_HEARTBEAT_PERIOD	MASTER_HOST	MASTER_LOG_FILE
MASTER_LOG_POS	MASTER_PASSWORD	MASTER_PORT
MASTER_RETRY_COUNT	MASTER_SERVER_ID	MASTER_SSL
MASTER_SSL_CA	MASTER_SSL_CAPATH	MASTER_SSL_CERT
MASTER_SSL_CIPHER	MASTER_SSL_CRL	MASTER_SSL_CRLPATH
MASTER_SSL_KEY	MASTER_SSL_VERIFY_SERVER_CERTR	MASTER_TLS_VERSION
MASTER_USER	MATCHR	MAXVALUER
MAX_CONNECTIONS_PER_HOUR	MAX_QUERIES_PER_HOUR	MAX_ROWS
MAX_SIZE	MAX_STATEMENT_TIME	MAX_UPDATES_PER_HOUR
MAX_USER_CONNECTIONS	MEDIUM	MEDIUMBLOBR
MEDIUMINTR	MEDIUMTEXTR	MEMORY
MERGE	MESSAGE_TEXT	MICROSECOND
MIDDLEINTR	MIGRATE	MINUTE
MINUTE_MICROSECONDR	MINUTE_SECONDR	MIN_ROWS
MODR	MODE	MODIFIESR
MODIFY	MONTH	MULTILINESTRING
MULTIPOINT	MULTIPOLYGON	MUTEX
MYSQL_ERRNO	NAME	NAMES
NATIONAL	NATURALR	NCHAR
NDB	NDBCLUSTER	NEVER
NEW	NEXT	NO
NODEGROUP	NONBLOCKING	NONE
NOTR	NO_WAIT	NO_WRITE_TO_BINLOGR
NULLR	NUMBER	NUMERICR
NVARCHAR	OFFSET	OLD_PASSWORD
ONR	ONE	ONLY
OPEN	OPTIMIZER	OPTIMIZER_COSTSR
OPTIONR	OPTIONALLYR	OPTIONS
ORR	ORDERR	OUTR
OUTERR	OUTFILER	OWNER
PACK_KEYS	PAGE	PARSER
PARSE_GCOL_EXPR	PARTIAL	PARTITIONR
PARTITIONING	PARTITIONS	PASSWORD
PHASE	PLUGIN	PLUGINS
PLUGIN_DIR	POINT	POLYGON
PORT	PRECEDES	PRECISIONR
PREPARE	PRESERVE	PREV
PRIMARYR	PRIVILEGES	PROCEDURER
PROCESSLIST	PROFILE	PROFILES
PROXY	PURGER	QUARTER
QUERY	QUICK	RANGER
READR	READSR	READ_ONLY
READ_WRITER	REALR	REBUILD
RECOVER	REDOFILE	REDO_BUFFER_SIZE
REDUNDANT	REFERENCESR	REGEXPR
RELAY	RELAYLOG	RELAY_LOG_FILE
RELAY_LOG_POS	RELAY_THREAD	RELEASER
RELOAD	REMOVE	RENAMER
REORGANIZE	REPAIR	REPEATR
REPEATABLE	REPLACER	REPLICATE_DO_DB
REPLICATE_DO_TABLE	REPLICATE_IGNORE_DB	REPLICATE_IGNORE_TABLE
REPLICATE_REWRITE_DB	REPLICATE_WILD_DO_TABLE	REPLICATE_WILD_IGNORE_TABLE
REPLICATION	REQUIRER	RESET
RESIGNALR	RESTORE	RESTRICTR
RESUME	RETURNR	RETURNED_SQLSTATE
RETURNS	REVERSE	REVOKER
RIGHTR	RLIKER	ROLLBACK
ROLLUP	ROTATE	ROUTINE
ROW	ROWS	ROW_COUNT
ROW_FORMAT	RTREE	SAVEPOINT
SCHEDULE	SCHEMAR	SCHEMASR
SCHEMA_NAME	SECOND	SECOND_MICROSECONDR
SECURITY	SELECTR	SENSITIVER
SEPARATORR	SERIAL	SERIALIZABLE
SERVER	SESSION	SETR
SHARE	SHOWR	SHUTDOWN
SIGNALR	SIGNED	SIMPLE
SLAVE	SLOW	SMALLINTR
SNAPSHOT	SOCKET	SOME
SONAME	SOUNDS	SOURCE
SPATIAL ®	SPECIFIC ®	SQL ®
SQLEXCEPTION ®	SQLSTATE ®	SQLWARNING ®
SQL_AFTER_GTIDS	SQL_AFTER_MTS_GAPS	SQL_BEFORE_GTIDS
SQL_BIG_RESULTR	SQL_BUFFER_RESULT	SQL_CACHE
SQL_CALC_FOUND_ROWSR	SQL_NO_CACHE	SQL_SMALL_RESULTR
SQL_THREAD	SQL_TSI_DAY	SQL_TSI_HOUR
SQL_TSI_MINUTE	SQL_TSI_MONTH	SQL_TSI_QUARTER
SQL_TSI_SECOND	SQL_TSI_WEEK	SQL_TSI_YEAR
SSLR	STACKED	START
STARTINGR	STARTS	STATS_AUTO_RECALC
STATS_PERSISTENT	STATS_SAMPLE_PAGES	STATUS
STOP	STORAGE	STOREDR
STRAIGHT_JOINR	STRING	SUBCLASS_ORIGIN
SUBJECT	SUBPARTITION	SUBPARTITIONS
SUPER	SUSPEND	SWAPS
SWITCHES	TABLER	TABLES
TABLESPACE	TABLE_CHECKSUM	TABLE_NAME
TEMPORARY	TEMPTABLE	TERMINATEDR
TEXT	THAN	THENR
TIME	TIMESTAMP	TIMESTAMPADD
TIMESTAMPDIFF	TINYBLOBR	TINYINTR
TINYTEXTR	TOR	TRAILINGR
TRANSACTION	TRIGGER ®	TRIGGERS
TRUE ®	TRUNCATE	TYPE
TYPES	UNCOMMITTED	UNDEFINED
UNDOR	UNDOFILE	UNDO_BUFFER_SIZE
UNICODE	UNINSTALL	UNIONR
UNIQUER	UNKNOWN	UNLOCKR
UNSIGNEDR	UNTIL	UPDATER
UPGRADE	USAGER	USER
USER	USER_RESOURCES	USE_FRM
USINGR	UTC_DATER	UTC_TIMER
UTC_TIMESTAMPR	VALIDATION	VALUE
VALUESR	VARBINARYR	VARCHARR
VARCHARACTERR	VARIABLES	VARYINGR
VIEW	VIRTUALR	WAIT
WARNINGS	WEEK	WEIGHT_STRING
WHENR	WHERER	WHILER
WITHR	WITHOUT	WORK
WRAPPER	WRITER	X509
XA	XID	XML
XORR	YEAR	YEAR_MONTHR
ZEROFILLR