MyBatis实现模糊查询的几种方式

时间：2021-02-18 11:17:45|栏目：JAVA代码|点击：次

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();
 }

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