Mysql 關於處理NULL值的相關函數和操做符

操做符

<=>

NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.
NULL安全的相等比較操做符。這個操做符和‘=’操做符同樣用來執行相等的比較,但返回1而不是NULL若是兩個操做數都是NULL,返回0而不是NULL若是隻有一個操做數是零。

mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
        -> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
        -> 1, NULL, NULL

For row comparisons, (a, b) <=> (x, y) is equivalent to:
(a <=> x) AND (b <=> y)

IS NULL

Tests whether a value is NULL.

mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
        -> 0, 0, 1

IS NOT NULL

Tests whether a value is not NULL.

mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
        -> 1, 1, 0

函數

ISNULL(expr)

If expr is NULL, ISNULL() returns 1, otherwise it returns 0.

mysql> SELECT ISNULL(1+1);
        -> 0
mysql> SELECT ISNULL(1/0);
        -> 1

ISNULL() can be used instead of = to test whether a value is NULL. (Comparing a value to NULL using = always yields NULL.)
The ISNULL() function shares some special behaviors with the IS NULL comparison operator. See the description of IS NULL.

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

mysql> SELECT IFNULL(1,0);
        -> 1
mysql> SELECT IFNULL(NULL,10);
        -> 10
mysql> SELECT IFNULL(1/0,10);
        -> 10
mysql> SELECT IFNULL(1/0,'yes');
        -> 'yes'

The default return type of IFNULL(expr1,expr2) is the more 「general」 of the two expressions, in the order STRING, REAL, or INTEGER.
IFNULL(expr1,expr2)的默認結果值爲兩個表達式中更加「通用」的一個，順序爲STRING、 REAL或 INTEGER。

 Consider the case of a table based on expressions or where MySQL must internally store a value returned by IFNULL() in a temporary table:
若是一個表是基於表達式的表或MySQL必須在一個臨時表中存儲一個IFNULL()的返回值：

mysql> CREATE TABLE tmp SELECT IFNULL(1,'test') AS test;
mysql> DESCRIBE tmp;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| test  | varbinary(4) | NO   |     |         |       |
+-------+--------------+------+-----+---------+-------+

In this example, the type of the test column is VARBINARY(4) (a string type).
在這個例子中，測試列的類型爲 VARBINARY(4) (a string type).

NULLIF(expr1,expr2)

Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

The return value has the same type as the first argument.

mysql> SELECT NULLIF(1,1);
        -> NULL
mysql> SELECT NULLIF(1,2);
        -> 1

COALESCE

用法

COALESCE(value,...)

說明

Returns the first non-NULL value in the list, or NULL if there are no non-NULL values.
返回參數列表裏邊第一個非空值，若是沒有非空值，則返回NULL

The return type of COALESCE() is the aggregated type of the argument types.

mysql> SELECT COALESCE(NULL,1);
        -> 1
mysql> SELECT COALESCE(NULL,NULL,NULL);
        -> NULL

mysql>SELECT COALESCE(A, B, C);
        -> A if A is non-NULL else B if B is non-NULL else C if C is non-NULL else NULL

參考資料

https://dev.mysql.com/doc/refman/8.0/en/func-op-summary-ref.html