MySQL null的一些易錯點
依據null-values,MySQL的值為null的意思只是代表沒有資料,null值和某種型別的零值是兩碼事,比如int型別的零值為0,字串的零值為””,但是它們依然是有資料的,不是null.
我們在儲存資料的時候,習慣性的把暫時沒有的資料記為null,表示當前我們無法提供有效的資訊.
不過使用null但是時候,需要我們注意一些問題.對此MySQL文件說明如下: problems-with-null
使用null的易錯點
下面我摘取MySQL官方給出的null的易錯點做講解.
對MySQL不熟悉的人很容易搞混null和零值
The concept of the NULL value is a common source of confusion for newcomers to SQL
比如下面這2句SQL產生的資料是獨立的
mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ('');
第一句SQL只是表示暫時不知道電話號碼是多少,第二句是電話號碼知道並且記錄為''
Both statements insert a value into the phone column,but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as “phone number is not known” and the meaning of the second can be regarded as “the person is known to have no phone,and thus no phone number.”
對null的邏輯判斷要單獨處理
對於是否為null的判斷必須使用專門的語法IS NULL,IS NOT NULL,IFNULL().
To help with NULL handling,you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.
如果你使用=判斷,那麼永遠是false
In SQL,the NULL value is never true in comparison to any other value,even NULL
To search for column values that are NULL,you cannot use an expr = NULL test. The following statement returns no rows,because expr = NULL is never true
比如你這樣寫,where後判斷的結果永不會是true:
SELECT * FROM my_table WHERE phone = NULL;
如果你使用null和其他資料做計算,那麼結果永遠是null,除非MySQL文件對某些操作做了額外的特殊說明
An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression
例如:
mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);
+------+--------+--------------------------+
| NULL | 1+NULL | CONCAT('Invisible',NULL) |
+------+--------+--------------------------+
| NULL | NULL | NULL |
+------+--------+--------------------------+
1 row in set (0.00 sec)
所以你要對null做邏輯判斷,還是乖乖的使用IS NULL
To look for NULL values,you must use the IS NULL test
對有null值的列做索引要額外預料到隱藏的細節
只有InnoDB,MyISAM,MEMORY 儲存引擎支援給帶有null值的列做索引
You can add an index on a column that can have NULL values if you are using the MyISAM,InnoDB,or MEMORY storage engine. Otherwise,you must declare an indexed column NOT NULL,and you cannot insert NULL into the column.
索引的長度會比普通索引大1,也就是略微耗記憶體點
Due to the key storage format,the key length is one greater for a column that can be NULL than for a NOT NULL column.
對null值做分組,去重,排序會被特殊對待
和上文講的=null永遠是false相反,這時null 被認為是相等的.
When using DISTINCT,GROUP BY,or ORDER BY,all NULL values are regarded as equal.
對null排序會被特殊對待
null值要麼被排在最前面,要麼最後面
When using ORDER BY,NULL values are presented first,or last if you specify DESC to sort in descending order.
聚合操作時null被忽略
Aggregate (group) functions such as COUNT(),MIN(),and SUM() ignore NULL values
例如count(*)不會統計值為null的資料.
The exception to this is COUNT(*),which counts rows and not individual column values. For example,the following statement produces two counts. The first is a count of the number of rows in the table,and the second is a count of the number of non-NULL values in the age column:
mysql> SELECT COUNT(*),COUNT(age) FROM person;
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