• When taking values, #{} the corresponding position in the extracted sql statement is placeholder?, ${} fills in the value.

  • {} set parameters to sql statements by precompiling. You can use sql injection.

  • The value taken out by ${} is directly spliced in the sql statement.
  • In most cases, use #{}.
  • Use ${} when placeholders are not supported by native jdbc

• javaType: usually needs to be set in some specific environment.

  • Some databases that do not support null values, such as Oracle

  • When passing values, indicate that jdbcType=NULL

    #{email,jdbcType=null}
    

  • You can also modify JDBC typefornull to NULL in the global configuration file

• When querying, if the return value type is list, set the type of elements in the list collection when setting the return value in the mapping file configuration.

• When querying the map of a record, set the return value to map when configuring in the mapping file.

  • Key is the column name and Value is the corresponding Value

• When querying multiple records and encapsulating them into a Map, if you need to specify a key, you need to set it through annotation.

  @MapKey("id")
  

ResultMap implements advanced result set mapping

• ResultMap can customize result set mapping rules.
• Only one ResultMap and ResultType can be used
• Add a class attribute to map each data in the database. And give the id (unique id) to the resultMap on the select tag

<resultMap id="MyGuild" type="com.boerk.mybatis.pojo.Guild">
   <id column="guild_id" property="guildID"></id>
   <result column="guild_name" property="guildName"></result>
   <result column="guild_pwd" property="guildPwd"></result>
   <result column="guild_job" property="guildJob"></result>
   <result column="guild_belief" property="guildBelief"></result>
   <result column="guild_influence" property="guildInfluence"></result>
   <result column="guild_specialty" property="guildSpecialty"></result>
   <result column="guild_level" property="guildLevel"></result>
</resultMap>
<select id="getGuildId" resultMap="MyGuild">

• When you join queries, you can use cascading attributes to encapsulate the result set.

  • When an entity class is a property of another entity class.

  • For example, deptNO and deptName are attributes of the dept object, and the dept object is an attribute of emp. Therefore, when packaging

    <result column="dept_name" property="dept.deptName"/>
    

  • Or use the association tag directly: you can specify the federated javabean object. (defined inside resultMap)

    <!--property="dept": Specify which is a federated object
    	javaType="com.boerk.batis.pojo.Dept": Specifies the type of Federated object.
    -->
    <association property="dept" javaType="com.boerk.batis.pojo.Dept"></association>
    

  • You can query step by step through the association tag

    <!--select="xx.xx"Indicates that the current property is a call select Results found by the specified method
    	column="d.id": Specifies which column value to pass to this method-->
    <association property="dept" select="xxx.xx" column="d.id"></association>
    

  • Delayed loading can be achieved by configuring lazyloading enabled = true and aggressiveLazyLoading=false in the global configuration. sql will be executed only when necessary to save resources.

  • Use the clooection tag to define the encapsulation rules of the attributes associated with the collection type.

    <resultMap id="hello" type="com.boerk.mybatis.pojo.Department">
        <id column="department_id" property="departmentId"></id>
        <result column="department_name" property="departmentName"></result>
        <collection property="list" ofType="com.boerk.mybatis.pojo.Employee">
             <id column="employee_id" property="employeeId"></id>
             <result column="last_name" property="lastName"></result>
             <result column="email" property="email"></result>
        </collection>
    </resultMap>
    

  • Discriminator: discriminator. mybatis can use discriminator to determine the value of a column, and then change the encapsulation behavior according to the value of a column

    • column: judge according to the value of this class
    • javaType: the type of this class
    • Judge by changing the value in value (similar to switch)