如何在 MySQL 查询中连接值并处理 Null 值?
mysqlmysqli database
您可以使用 CONCAT() 方法连接值，而 IFNULL() 方法用于处理 NULL 值。语法如下:
SELECT
CONCAT('anyStringValue:',IFNULL(yourColumnName, 'anyStringValue’)) AS anyVariableName
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 ConcatValues -> ( -> Id int NOT NULL AUTO_INCREMENT, -> FirstName varchar(20), -> MiddleName varchar(20), -> LastName varchar(20), -> PRIMARY KEY(Id) -> ); Query OK, 0 rows affected (0.62 sec)
使用 insert 命令在表中插入一些记录。 查询语句如下:
mysql> insert into ConcatValues(FirstName,MiddleName,LastName) values('John' ,NULL ,'Smith');
Query OK, 1 row affected (0.24 sec)
mysql> insert into ConcatValues(FirstName,MiddleName,LastName) values('Carol' ,NULL ,'Taylor');
Query OK, 1 row affected (0.16 sec)
mysql> insert into ConcatValues(FirstName,MiddleName,LastName) values('David' ,NULL ,'Miller');
Query OK, 1 row affected (0.23 sec)
使用 select 语句显示表中的所有记录。查询如下:
mysql> select *from ConcatValues;
输出结果如下:
+----+-----------+------------+----------+ | Id | FirstName | MiddleName | LastName | +----+-----------+------------+----------+ | 1 | Mary | Elizabeth | Smith | | 2 | John | NULL | Smith | | 3 | Carol | NULL | Taylor | | 4 | David | NULL | Miller | +----+-----------+------------+----------+ 4 rows in set (0.00 sec)
这是处理 NULL 和连接值的查询:
mysql> select
-> concat('The middle name is:',IFNULL(MiddleName, 'Not Available')) AS MiddleName
-> from ConcatValues;
输出结果如下:
+----------------------------------+ | MiddleName | +----------------------------------+ | The middle name is:Elizabeth | | The middle name is:Not Available | | The middle name is:Not Available | | The middle name is:Not Available | +----------------------------------+ 4 rows in set (0.00 sec)
