MyBatis实现模糊查询的几种方式

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

public static void main(String[] args) {
 try {
  int count = 500;

long begin = System.currentTimeMillis();
  testString(count);
  long end = System.currentTimeMillis();
  long time = end - begin;
  System.out.println("String 方法拼接"+count+"次消耗时间：" + time + "毫秒");

begin = System.currentTimeMillis();
  testStringBuilder(count);
  end = System.currentTimeMillis();
  time = end - begin;
  System.out.println("StringBuilder 方法拼接"+count+"次消耗时间：" + time + "毫秒");

} catch (Exception e) {
  e.printStackTrace();
 }

}

private static String testString(int count) {
 String result = "";

for (int i = 0; i < count; i++) {
  result += "hello ";
 }

return result;
}

private static String testStringBuilder(int count) {
 StringBuilder sb = new StringBuilder();

for (int i = 0; i < count; i++) {
  sb.append("hello");
 }

return sb.toString();
}

来源：https://blog.csdn.net/lonely_dog/article/details/74171314