mybatis从使用到了解(七)_mybatis动态sql的妙用

动态SQL

在使用JDBC拼接SQL的时候，经常要确保不能完了必要的空格，对于的逗号，而mybatis的动态SQL则完美的解决了这些问题。本文只介绍利用mybatis的动态SQL解决常见的SQL拼接问题。
mybatis的动态sql包含一下内容：

• if
• choose,when,otherwise
• trim,where,set
• foreach
• bind

解决where后SQL条件判断问题

    select * from student where
    
        student_id = #{studentId}
    
    
        and student_age = #{studentAge}
    
    
        and student_phone = #{studentPhone}
    

在上名的sql中，如果三个if条件全为空，则最后拼接的sql为：

select * from student where

如果第一个为判断为空，则最后拼接的sql为：

select * from student where and student_age = #{studentAge} and student_phone = #{studentPhone}

上面拼接的两个sql语法都存在问题，只需要利用一点小技巧就能解决这个问题。如下，利用标签，mybatis会自动处理上面的问题。

    select  *   from student
    
        
            student_id = #{studentId}
        
        
            and student_age = #{studentAge}
        
        
            and student_phone = #{studentPhone}
        
    

也可以利用trim来解决

    select  *  from student
    
        
            student_id = #{studentId}
        
        
            and student_age = #{studentAge}
        
        
            and student_phone = #{studentPhone}
        
    

利用或解决update中set逗号问题

    update student
    student_name = #{studentName},
    student_age = #{studentAge},
    student_name = student_phone = #{studentPhone},
    where student_id = #{studentId}

从上面可以看出，set始终会多一个逗号。解决方案如下：

    update student
    
        student_name = #{studentName},
        student_age = #{studentAge},
        student_name = student_phone = #{studentPhone},
    
    where student_id = #{studentId}

或者

    update student
    
        student_name = #{studentName},
        student_age = #{studentAge},
        student_name = student_phone = #{studentPhone},
    
    where student_id = #{studentId}

利用foreach查询

    select
    
    from student where student_id in
    
        #{item}