MyBatis | Dynamic SQL | where, trim

DB table

test_table

id	string	number
1	hoge	100
2	hoge	200
3	fuga	200
4	piyo	400

Source code

sample_mapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="sample.mybatis">

  <select id="selectTest" resultType="sample.mybatis.TestTable">
    select *
      from test_table
    <where>
      <if test="string != null">
        and string = #{string}
      </if>
      <if test="number != null">
        and number = #{number}
      </if>
    </where>
  </select>
</mapper>

Main.java

package sample.mybatis;

import java.io.InputStream;
import java.util.HashMap;
import java.util.Map;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class Main {

    public static void main(String[] args) throws Exception {
        try (InputStream in = Main.class.getResourceAsStream("/mybatis-config.xml")) {
            SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);

            try (SqlSession session = factory.openSession()) {
                // Run with an empty parameter object.
                Map<String, Object> param = new HashMap<>();
                selectAndPrint(session, param);

                // Run with only string specified as a parameter.
                param = new HashMap<>();
                param.put("string", "hoge");
                selectAndPrint(session, param);

                // Run with both string and number specified as parameters.
                param = new HashMap<>();
                param.put("string", "hoge");
                param.put("number", 200);
                selectAndPrint(session, param);
            }
        }
    }

    private static void selectAndPrint(SqlSession session, Map<String, Object> param) {
        session
            .selectList("sample.mybatis.selectTest", param)
            .forEach(System.out::println);
        System.out.println();
    }
}

Execution result

[DEBUG] s.m.selectTest  - ==>  Preparing: select * from test_table 
[DEBUG] s.m.selectTest  - ==> Parameters: 
[DEBUG] s.m.selectTest  - <==      Total: 4
TestTable [id=1, string=hoge, number=100]
TestTable [id=2, string=hoge, number=200]
TestTable [id=3, string=fuga, number=200]
TestTable [id=4, string=piyo, number=300]

[DEBUG] s.m.selectTest  - ==>  Preparing: select * from test_table WHERE string = ? 
[DEBUG] s.m.selectTest  - ==> Parameters: hoge(String)
[DEBUG] s.m.selectTest  - <==      Total: 2
TestTable [id=1, string=hoge, number=100]
TestTable [id=2, string=hoge, number=200]

[DEBUG] s.m.selectTest  - ==>  Preparing: select * from test_table WHERE string = ? and number = ? 
[DEBUG] s.m.selectTest  - ==> Parameters: hoge(String), 200(Integer)
[DEBUG] s.m.selectTest  - <==      Total: 1
TestTable [id=2, string=hoge, number=200]

Explanation

When using the <where> tag, a WHERE clause is added at the beginning only if one of the child elements produces text.
Also, if the text inside the <where> tag starts with AND or OR, that condition connector is removed automatically.
Writing this behavior with only tags such as <if> would become fairly complex, but the <where> tag makes it simple.

Replacing it with the trim tag

The definition that uses <where> above can be rewritten with the <trim> tag as follows.

sample_mapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="sample.mybatis">

  <select id="selectTest" resultType="sample.mybatis.TestTable">
    select *
      from test_table
    <trim prefix="where" prefixOverrides="AND|OR">
      <if test="string != null">
        and string = #{string}
      </if>
      <if test="number != null">
        and number = #{number}
      </if>
    </trim>
  </select>
</mapper>

Add the string to prepend in the prefix attribute.
In the prefixOverrides attribute, specify strings to remove from the beginning, separated by pipes (|).