Dynamic SQL

Posted 2022-02-12 Beyond meat

 

Mybatis 的特色功能就是动态语句。在使用 JDBC 的过程中，根据条件进行 SQL 的拼接麻烦且易错。 MyBatis 动态 SQL 的出现， 顺利地解决了这个麻烦。

MyBatis通过 OGNL 和 JSTL 来进行动态 SQL 的使用。

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One of the most powerful features of MyBatis has always been its Dynamic SQL capabilities. If you have any experience with JDBC or any similar framework, you understand how painful it is to conditionally concatenate[kɑnˈkæt(ə)ˌneɪt] adj.连锁状的 v.使(成串地)连结[衔接]起来strings of SQL together, making sure not to forget spaces or to omit a comma at the end of a list of columns. Dynamic SQL can be downright 完全的/地 painful to deal with.

While working with Dynamic SQL will never be a party, MyBatis certainly improves the situation with a powerful Dynamic SQL language that can be used within any mapped SQL statement.

The Dynamic SQL elements should be familiar to anyone who has used JSTL(JSP Standard Tag Library) or any similar XML based text processors处理器. In previous versions of MyBatis, there were a lot of elements to know and understand. MyBatis 3 greatly improves upon this, and now there are less than half of those elements to work with. MyBatis employs powerful OGNL (Object Graph Navigation Language) based expressions to eliminate most of the other elements:

• if
• choose (when, otherwise)
• trim (where, set)
• foreach

  if

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

  <select id="findActiveBlogWithTitleLike" resultType="Blog">
        SELECT * FROM BLOG WHERE state = ‘ACTIVE’
        <if test="title != null">
              AND title like #title
        </if>
  </select>

  This statement would provide an optional text search type of functionality n.实用. If you passed in no title, then all active Blogs would be returned. But if you do pass in a title, it will look for a title like that (for the keen eyed, yes（adv(回答问话、招呼等)是） in this case your parameter value would need to include any masking or wildcard characters).

  What if we wanted to optionally search by title and author? First, I’d change the name of the statement to make more sense. Then simply add another condition.

  <select id="findActiveBlogLike" resultType="Blog">
    SELECT * FROM BLOG WHERE state = ‘ACTIVE’
    <if test="title != null">
      AND title like #title
    </if>
    <if test="author != null and author.name != null">
      AND author_name like #author.name
    </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. Similar to a switch statement in Java, MyBatis offers a choose element.

  Let’s use the example above, but now let’s search only on title if one is provided, then only by author if one is provided. If neither is provided, let’s only return featured blogs (perhaps a strategically list selected by administrators, instead of returning a huge meaningless list of random blogs).

<select id="findActiveBlogLike" resultType="Blog">
  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

The previous examples have been conveniently dancing around a notorious dynamic SQL challenge. Consider what would happen if we return to our "if" example, but this time we make "ACTIVE = 1" a dynamic condition as well.

<select id="findActiveBlogLike" resultType="Blog">
  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>
</select>

What happens if none of the conditions are met? You would end up with SQL that looked like this

SELECT * FROM BLOG
WHERE
AND title like ‘someTitle’

This would also fail. This problem is not easily solved with conditionals, and if you’ve ever had to write it, then you likely never want to do so again.

MyBatis has a simple answer that will likely work in 90% of the cases. And in cases where it doesn’t, you can customize it so that it does. With one simple change, everything works fine:

<select id="findActiveBlogLike" resultType="Blog">
  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>

The where element knows to only insert "WHERE" if there is any content returned by the containing tags. Furthermore, if that content begins with "AND" or "OR", it knows to strip it off.

If the where element does not behave exactly as you like, 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>

The prefixOverrides attribute takes a pipe delimited（delimit [diˈlɪmɪt] v.界定） list of text to override, where whitespace is relevant. The result is the removal of anything specified in the prefixOverrides attribute, and the insertion of anything in the prefix attribute.

There is a similar solution for dynamic update statements called set. The set element can be used to dynamically include columns to update, and leave out others. For example:

<update id="updateAuthorIfNecessary">
  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>

Here, the set element will dynamically prepend预置(pend v.吊着) the SET keyword, and also eliminate any extraneous([ɪkˈstreɪniəs]adj.无关的) commas that might trail the value assignments after the conditions are applied.

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

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

Notice that in this case we’re overriding a suffix, while we’re still appending a prefix.

foreach

Another common necessity for dynamic SQL is the need to iterate([\'ɪtəreɪt] ) over a collection, often to build an IN condition. For example:

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  <where>
    <foreach item="item" index="index" collection="list"
        open="ID in (" separator="," close=")" nullable="true">
          #item
    </foreach>
  </where>
</select>

The foreach element is very powerful, and allows you to specify a collection, declare item and index variables that can be used inside the body of the element. It also allows you to specify opening and closing strings, and add a separator to place in between iterations. The element is smart in that it won’t accidentally append extra separators.

NOTE You can pass any Iterable object (for example List, Set, etc.), as well as any Map or Array object to foreach as collection parameter. When using an Iterable or Array, index will be the number of current iteration and value item will be the element retrieved in this iteration. When using a Map (or Collection of Map.Entry objects(A map entry (key-value pair). The Map.entrySet method returns a collection-view of the map, whose elements are of this class. The only way to obtain a reference to a map entry is from the iterator of this collection-view. These Map.Entry objects are valid only for the duration of the iteration; more formally, the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the setValue operation on the map entry.)), index will be the key object and item will be the value object.

This wraps up the discussion regarding the XML configuration file and XML mapping files. The next section will discuss the Java API in detail, so that you can get the most out of the mappings that you’ve created.

script

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

   @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. For example:

<select id="selectBlogsLike" resultType="Blog">
  <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. Have a look at the following example:

<insert id="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

Starting from version 3.2 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 
  ParameterHandler createParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql);
  SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType);
  SqlSource createSqlSource(Configuration configuration, String script, Class<?> parameterType);

Once you have your custom language driver you can set it to be the default by configuring it in the mybatis-config.xml file:

<typeAliases>
  <typeAlias type="org.sample.MyLanguageDriver" alias="myLanguage"/>
</typeAliases>
<settings>
  <setting name="defaultScriptingLanguage" value="myLanguage"/>
</settings>

Instead of changing the default, you can specify the language for an specific statement by adding the lang attribute as follows:

<select id="selectBlog" lang="myLanguage">
  SELECT * FROM BLOG
</select>

Or, in the case you are using mappers, using the @Lang annotation:

public interface Mapper 
  @Lang(MyLanguageDriver.class)
  @Select("SELECT * FROM BLOG")
  List<Blog> selectBlog();

NOTE You can use Apache Velocity([vəˈlɑsəti] 高速) as your dynamic language. Have a look at the MyBatis-Velocity project for the details.

All the xml tags you have seen in the previous sections are provided by the default MyBatis language that is provided by the driver org.apache.ibatis.scripting.xmltags.XmlLanguageDriver which is aliased( [ˈeɪliəs]adv. 又名，化名（尤指演员或罪犯）n. （尤指罪犯所用的）化名，别名；（文件、互联网地址等用的）别名，假名；假频伪信号v. 错误识别（信号频率），使失真) as xml.