Java Mybatis - Dynamic SQL

One of the most powerful features of MyBatis has always been its Dynamic SQL capabilities.

if

The most common thing to do in dynamic SQL is conditionally include a part of a where clause.

<select >
  SELECT * FROM BLOG
  WHERE state = ‘ACTIVE’
  <if test="title != null">
    AND title like #{title}
  </if>
</select>

choose, when, otherwise

Sometimes we don’t want all of the conditionals to apply, instead we want to choose only one case among many options.

<select >
 SELECT * FROM BLOG WHERE state = ‘ACTIVE’
 <choose>
   <when test="title != null">
     AND title like #{title}
   </when>
   <when test="author != null and author.name != null">
     AND author_name like #{author.name}
   </when>
   <otherwise>
     AND featured = 1
   </otherwise>
 </choose>
</select>

trim, where, set

where, With one simple change, everything works fine.

<select >
  SELECT * FROM BLOG
  <where>
    <if test="state != null">
         state = #{state}
    </if>
    <if test="title != null">
        AND title like #{title}
    </if>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
  </where>
</select>

you can customize it by defining your own trim element. For example, the trim equivalent to the where element is:

<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
</trim>

There is a similar solution for dynamic update statements called set.

<update >
  update Author
    <set>
      <if test="username != null">username=#{username},</if>
      <if test="password != null">password=#{password},</if>
      <if test="email != null">email=#{email},</if>
      <if test="bio != null">bio=#{bio}</if>
    </set>
  where id=#{id}
</update>

If you’re curious about what the equivalent trim element would look like, here it is:

<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

foreach

Another common necessity for dynamic SQL is the need to iterate over a collection, often to build an IN condition.

<select >
  SELECT *
  FROM POST P
  <where>
    <foreach item="item" index="index" collection="list"
        open="ID in (" separator="," close=")" nullable="true">
          #{item}
    </foreach>
  </where>
</select>

script

For using dynamic SQL in annotated mapper class, script element can be used.

    @Update({"<script>",
      "update Author",
      "  <set>",
      "    <if test='username != null'>username=#{username},</if>",
      "    <if test='password != null'>password=#{password},</if>",
      "    <if test='email != null'>email=#{email},</if>",
      "    <if test='bio != null'>bio=#{bio}</if>",
      "  </set>",
      "where id=#{id}",
      "</script>"})
    void updateAuthorValues(Author author);

bind

The bind element lets you create a variable out of an OGNL expression and bind it to the context.

<select >
  <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
  SELECT * FROM BLOG
  WHERE title LIKE #{pattern}
</select>

Multi-db vendor support

If a databaseIdProvider was configured a "_databaseId" variable is available for dynamic code, so you can build different statements depending on database vendor.

<insert >
  <selectKey keyProperty="id" resultType="int" order="BEFORE">
    <if test="_databaseId == 'oracle'">
      select seq_users.nextval from dual
    </if>
    <if test="_databaseId == 'db2'">
      select nextval for seq_users from sysibm.sysdummy1"
    </if>
  </selectKey>
  insert into users values (#{id}, #{name})
</insert>

Pluggable Scripting Languages For Dynamic SQL

MyBatis supports pluggable scripting languages, so you can plug a language driver and use that language to write your dynamic SQL queries.
You can plug a language by implementing the following interface:

public interface LanguageDriver {
  ......
}