mysql 索引列为Null的走不走索引及null在统计时的问题

要尽可能地把字段定义为 NOT NULL，即使应用程序无须保存 NULL（没有值），也有许多表包含了可空列（Nullable Column）
这仅仅是因为它为默认选项。除非真的要保存 NULL，否则就把列定义为 NOT NULL

MySQL难以优化引用了可空列的查询,它会使索引、索引统计和值更加复杂。
可空列需要更多的储存空间，还需要在MySQL内部进行特殊处理。当可空列被索引的时候，
每条记录都需要一个额外的字节，还可能导致 MyISAM 中固定大小的索引(例如一个整数列上的索引)变成可变大小的索引。
即使要在表中储存「没有值」的字段，还是有可能不使用 NULL 的，考虑使用 0、特殊值或空字符串来代替它。
把 NULL 列改为 NOT NULL 带来的性能提升很小，所以除非确定它引入了问题，否则就不要把它当作优先的优化措施。
然后，如果计划对列进行索引，就要尽量避免把它设置为可空,虽然在mysql里 Null值的列也是走索引的

 mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
|         0 |             1 |
+-----------+---------------+
1 row in set

mysql> 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 |
+-----------+---------------+------------+----------------+

测试

CREATE TABLE `test_null` (
  `id` int(11) DEFAULT NULL,
  `mark` varchar(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

create procedure test_null(in num int)
BEGIN
DECLARE i int;
set i=1;
while (i<num)
DO
  if mod(i,10)!=0 then
     insert into test_null values (i,concat('aaa',i));
   else
     insert into test_null values (null,concat('aaa',i));
   end if;
set i=i+1;
END while;
END;

call test_null(10000);

mysql> select count(*) from test_null;
+----------+
| count(*) |
+----------+
|     9999 |
+----------+
没加任何索引时
mysql> explain SELECT * from test_null WHERE id is null;
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | test_null | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 10105 |       10 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
在id上加普通索引
create index idx_test_null on test_null(id);
mysql> explain SELECT * from test_null WHERE id is null;
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
| id | select_type | table     | partitions | type | possible_keys | key           | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | test_null | NULL       | ref  | idx_test_null | idx_test_null | 5       | const |  999 |      100 | Using index condition |
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
null值也是走索引的

 null在count统计时时的问题

create table test (id int,val int);
INSERT INTO `test` VALUES ('', '');
INSERT INTO `test` VALUES ('', '');
INSERT INTO `test` VALUES ('', '');
INSERT INTO `test` VALUES ('', '');
INSERT INTO `test` VALUES ('', '');

1条语句统计id=1,id=2的个数

一般错误写法

select count(id=1) ,count(id=2) from test;

mysql> select count(id=1) ,count(id=2) from test;
+-------------+-------------+
| count(id=1) | count(id=2) |
+-------------+-------------+
|           5 |           5 |
+-------------+-------------+

需要注意count只不会统计null的列，0的会统计

mysql> select 1 or null as or1,1 and null as and1 ,0 and null as and0 ,0 or null as null0;
+------+------+------+-------+
| or1  | and1 | and0 | null0 |
+------+------+------+-------+
|    1 | NULL |    0 |  NULL |
+------+------+------+-------+

mysql> select id=1 ,id=2 from test;
+------+------+
| id=1 | id=2 |
+------+------+
|    1 |    0 |
|    1 |    0 |
|    0 |    1 |
|    0 |    1 |
|    0 |    1 |
+------+------+

要把0值变为null,count时不计算即可

mysql> select count(id=1) ,count(id=2) from test;
+-------------+-------------+
| count(id=1) | count(id=2) |
+-------------+-------------+
|           5 |           5 |
+-------------+-------------+
mysql> select id=1 or null,id=2 or null from test;
+--------------+--------------+
| id=1 or null | id=2 or null |
+--------------+--------------+
|            1 |         NULL |
|            1 |         NULL |
|         NULL |            1 |
|         NULL |            1 |
|         NULL |            1 |
+--------------+--------------+

所以正确的写法是

mysql> select count(id=1 or null),count(id=2 or null) from test;
+---------------------+---------------------+
| count(id=1 or null) | count(id=2 or null) |
+---------------------+---------------------+
|                   2 |                   3 |
+---------------------+---------------------+
1 row in set (0.00 sec)

select id,count(id) from test where id in(1,2) GROUP BY id

常数与null的运算

DROP TABLE IF EXISTS `test1`;
CREATE TABLE `test1` (
  `id` int(11) DEFAULT NULL,
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

-- ----------------------------
-- Records of test1
-- ----------------------------
INSERT INTO `test1` VALUES ('', '', '');
INSERT INTO `test1` VALUES ('', '', null);
INSERT INTO `test1` VALUES ('', '', '');
INSERT INTO `test1` VALUES ('', '', null);
INSERT INTO `test1` VALUES ('', null, null);

查询 id,a-b的数量(剩余计算)

错误写法

mysql> SELECT id ,(a-b) as remain from test1;
+------+--------+
| id   | remain |
+------+--------+
|    1 |      4 |
|    2 |   NULL |
|    3 |      2 |
|    4 |   NULL |
|    5 |   NULL |
+------+--------+

正确写法

mysql> SELECT id ,(IFNULL(a,0)-IFNULL(b,0)) as remain  from test1;
+------+--------+
| id   | remain |
+------+--------+
|    1 |      4 |
|    2 |      6 |
|    3 |      2 |
|    4 |      7 |
|    5 |      0 |
+------+--------+