mybatis _parameter _databaseId

Mybatis 內置參數之_parameter和_databaseId

1、如果一個參數，那麼_parameter就代表它

/**
* Sql語句查詢
*
* @param sql
* @return List<HashMap<String, Object>>
*/
List<HashMap<String, Object>> selectBySql(String sql);

<select id="selectBySql" parameterType="java.lang.String" resultType="java.util.HashMap">
    ${_parameter}
</select>

 

2、_databaseId就是代表當前數據庫的別名

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
    <databaseIdProvider type="DB_VENDOR">
        <property name="MySQL" value="mysql"/>
        <property name="Oracle" value="oracle"/> 
  </databaseIdProvider>
</configuration>

// 此時mybatis中內置的參數_databaseId中保存了用戶所指定的對應的數據庫廠商標識。
<select id="selectUsrs" databaseId="mysql" resultType="com.heiketu.pojo.Users">
    <if test="_databaseId == 'mysql'">
        select * from usrs where id = 2
    </if>
</select>
<select id="selectUsrs" databaseId="oracle" resultType="com.heiketu.pojo.Users">
    <if test="_databaseId == 'oracle'">
        select * from usrs where id = 2
    </if>
</select>