@Mapkey usage instructions

One 、 Preface

@MapKey  yes MyBatis Comments on the framework , Role is to List The result set is converted to key-value Formal Map Result set , It is convenient to quickly query the specified results from the result set .

1.1、 usage

1.1.1、dao layer

@MapKey("id")
Map getStudents();

1.1.2、xml layer

<!-- Get basic information of students -->
<select id="getStudents" resultType="map">
    SELECT id, idCard, name FROM students
</select>

1.2、 effect

Before using

After using

  Two 、 matters needing attention

2.1、 Be careful key The type of

It is worth noting that , adopt @MapKey The comment returns Map, Its key The type of is consistent with the specified field type .

for example ： Appoint id As Map Of key,id by int type , Then the Map Of key Also for the integer type , If you pass String Type of key To get value, You can't get .

 2.2 、 Solution

2.2.1、 Keep it as it is.

If you don't need to use it externally , You can keep it as it is , Just pay attention key The type of .

2.2.2、 Unified use String As Key

Directly through mysql function , Modify the returned data type .

<!-- Get basic information of students -->
<select id="getStudents" resultType="map">
    SELECT Cast(id as char) AS id, idCard, name FROM students
</select>

3、 ... and 、 Reference article

@MapKey Use _make_888 The blog of -CSDN Blog [email protected]