当MySQL中输入为null时，为date_format返回null？

IF()当输入在MySQL中为null时，使用函数为date_format返回null。语法如下-

SELECT IF(yourDateColumnName,date_format(yourDateColumnName, '%d/%m/%Y'),NULL) FROM yourTableName;

为了理解上述语法，让我们创建一个表。创建表的查询如下-

mysql> create table returnNullWhenInputIsNullDemo
   -> (
   -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> ShippingDate datetime
   -> );

现在，您可以使用insert命令在表中插入一些记录。查询如下-

mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values('2019-01-21');
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values(NULL);
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values('2018-03-25');
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values(NULL);
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values(NULL);
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values('2012-04-23');
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values('2018-12-01');
mysql> insert into returnNullWhenInputIsNullDemo(ShippingDate) values('2016-11-13');

使用select语句显示表中的所有记录。查询如下-

mysql> select *from returnNullWhenInputIsNullDemo;

以下是输出-

+----+---------------------+
| Id | ShippingDate        |
+----+---------------------+
| 1  | 2019-01-21 00:00:00 |
| 2  | NULL                |
| 3  | 2018-03-25 00:00:00 |
| 4  | NULL                |
| 5  | NULL                |
| 6  | 2012-04-23 00:00:00 |
| 7  | 2018-12-01 00:00:00 |
| 8  | 2016-11-13 00:00:00 |
+----+---------------------+
8 rows in set (0.00 sec)

这是在MySQL中输入为null时为date_format返回null的查询-

mysql> SELECT IF(ShippingDate,date_format(ShippingDate, '%d/%m/%Y'),NULL) FROM returnNullWhenInputIsNullDemo;

以下是输出-

+-------------------------------------------------------------+
| IF(ShippingDate,date_format(ShippingDate, '%d/%m/%Y'),NULL) |
+-------------------------------------------------------------+
| 21/01/2019                                                  |
| NULL                                                        |
| 25/03/2018                                                  |
| NULL                                                        |
| NULL                                                        |
| 23/04/2012                                                  |
| 01/12/2018                                                  |
| 13/11/2016                                                  |
+-------------------------------------------------------------+
8 rows in set (0.05 sec)