When using MyBatis for database operations, it is sometimes necessary to deal with double-layer nested data structures. In this case, we can use MyBatis' foreach tag to solve the problem. This guide will provide detailed descriptions on how to deal with double-layer nesting problems in MyBatis and provide two examples.
1. Nested Query
Example 1: Query the user and its associated orders
Suppose we have two tables: user and order, and one user can have multiple orders. We want to query all users and their associated order information.
First, we need to define two query statements in the Mapper file of MyBatis, one for querying the user and the other for querying the order. We can then use the foreach tag to execute these two query statements in a nested manner.
<!-- Query user -->
<select resultType="User">
SELECT * FROM user
</select>
<!-- Inquiry of orders -->
<select resultType="Order">
SELECT * FROM order WHERE user_id = #{userId}
</select>Next, define a new query statement in the Mapper file and use the foreach tag to nest the above two query statements.
<select resultMap="UserWithOrdersResultMap">
SELECT * FROM user
</select>
<resultMap type="User">
<id property="id" column="id"/>
<result property="name" column="name"/>
<collection property="orders" ofType="Order">
<id property="id" column="order_id"/>
<result property="amount" column="amount"/>
</collection>
</resultMap>In the above example, we used UserWithOrdersResultMap to map the query results. There is a List<Order> attribute orders in the User class, which is used to store user order information.
Finally, call the getUserWithOrders method in Java code to get the user and its associated order information.
User user = ("getUserWithOrders");
2. Nested Insert
Example 2: Insert the user and its associated order
Suppose we have two tables: user and order, and one user can have multiple orders. We want to insert a user and its associated order information.
First, we need to define two insert statements in the Mapper file of MyBatis, one for inserting the user and the other for inserting the order. We can then use the foreach tag to execute these two insert statements in nested.
<!-- Insert the user -->
<insert parameterType="User">
INSERT INTO user (name) VALUES (#{name})
</insert>
<!-- Insert an order -->
<insert parameterType="Order">
INSERT INTO order (user_id, amount) VALUES (#{userId}, #{amount})
</insert>Next, define a new insert statement in the Mapper file and use the foreach tag to nest the above two insert statements.
<insert parameterType="User">
INSERT INTO user (name) VALUES (#{name})
<foreach collection="orders" item="order" separator=";">
INSERT INTO order (user_id, amount) VALUES (#{id}, #{})
</foreach>
</insert>
In the above example, we use the List<Order> attribute orders of the User class to store the user's order information.
Finally, call the insertUserWithOrders method in Java code to insert the user and its associated order information.
User user = new User();
("John");
Order order1 = new Order();
(100);
Order order2 = new Order();
(200);
((order1, order2));
("insertUserWithOrders", user);The above is a complete guide to dealing with the problem of foreach double-layer nesting in MyBatis. By using foreach tags, we can easily handle double-layer nested data structures, implementing complex query and insert operations.
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