在学习MyBatis过程中想实现模糊查询，可惜失败了。后来上百度上查了一下，算是解决了。记录一下MyBatis实现模糊查询的几种方式。
　　数据库表名为test_student,初始化了几条记录，如图：
　　
　　
　　起初我在MyBatis的mapper文件中是这样写的：

    <select id="searchStudents" resultType="com.example.entity.StudentEntity"parameterType="com.example.entity.StudentEntity">SELECT * FROM test_student<where><if test="age != null and age != '' and compare != null and compare != ''">age${compare}#{age}</if><if test="name != null and name != ''">AND name LIKE '%#{name}%'</if><if test="address != null and address != ''">AND address LIKE '%#{address}%'</if></where>ORDER BY id</select>

写完后自我感觉良好，很开心的就去跑程序了，结果当然是报错了：

　　经百度得知，这么写经MyBatis转换后(‘%#{name}%’)会变为(‘%?%’),而(‘%?%’)会被看作是一个字符串，所以Java代码在执行找不到用于匹配参数的 ‘?’ ,然后就报错了。

解决方法

1.用${…}代替#{…}

   <select id="searchStudents" resultType="com.example.entity.StudentEntity"parameterType="com.example.entity.StudentEntity">SELECT * FROM test_student<where><if test="age != null and age != '' and compare != null and compare != ''">age${compare}#{age}</if><if test="name != null and name != ''">AND name LIKE '%${name}%'</if><if test="address != null and address != ''">AND address LIKE '%${address}%'</if></where>ORDER BY id</select>

查询结果如下图：

　　注：使用${…}不能有效防止SQL注入，所以这种方式虽然简单但是不推荐使用！！！

2.把’%#{name}%’改为”%”#{name}”%”

   <select id="searchStudents" resultType="com.example.entity.StudentEntity"parameterType="com.example.entity.StudentEntity">SELECT * FROM test_student<where><if test="age != null and age != '' and compare != null and compare != ''">age${compare}#{age}</if><if test="name != null and name != ''">AND name LIKE "%"#{name}"%"</if><if test="address != null and address != ''">AND address LIKE "%"#{address}"%"</if></where>ORDER BY id</select>

查询结果：

3.使用sql中的字符串拼接函数

   <select id="searchStudents" resultType="com.example.entity.StudentEntity"parameterType="com.example.entity.StudentEntity">SELECT * FROM test_student<where><if test="age != null and age != '' and compare != null and compare != ''">age${compare}#{age}</if><if test="name != null and name != ''">AND name LIKE CONCAT(CONCAT('%',#{name},'%'))</if><if test="address != null and address != ''">AND address LIKE CONCAT(CONCAT('%',#{address},'%'))</if></where>ORDER BY id</select>

查询结果：

4.使用标签

   <select id="searchStudents" resultType="com.example.entity.StudentEntity"parameterType="com.example.entity.StudentEntity"><bind name="pattern1" value="'%' + _parameter.name + '%'" /><bind name="pattern2" value="'%' + _parameter.address + '%'" />SELECT * FROM test_student<where><if test="age != null and age != '' and compare != null and compare != ''">age${compare}#{age}</if><if test="name != null and name != ''">AND name LIKE #{pattern1}</if><if test="address != null and address != ''">AND address LIKE #{pattern2}</if></where>ORDER BY id</select>

查询结果：

5.在Java代码中拼接字符串
　　这个方法没试过，就不贴代码和结果了。

————2017.07.03