Why is the value queried in mybatis 3 tables but the value is empty when obtained?

select (field_a, field_b, field_c, field_d,null,null) from test_a, test_b where ..

That is, there is no need to copy the bai

You can directly set the field to null.

zhi to dao up.

Just select those fields.

This can be solved as follows: select field name 1 + isnull (field name 2, 0) as show field name from table name, and when field name 2 is null, the system will treat it as if it is performing.

When you return a record with 0 rows, you can just get the result and determine whether it is null.

If you want to execute this SQL statement without passing in parameters, try using and.

Don't limit yourself to baseresultmap, just create a new class that receives the field value of the corresponding type.

According to debug 11-15 23:39:50,458 <=total: 1 (:139).

You can see that there are query results, but there is no corresponding resulttype= when assigning the value"teacher"Here the teacher path prefix is added, like.

Unless you have an alias defined in the front.

Which statement reports a null exception?。。。

First of all, mybatis itself does not determine whether it is a multi-table query, and there is no difference between a multi-table query and a single-table query, you need an object to receive the return value.

For example: select name, grade from table 1 u, table 2 p where = . Here you find the two fields name and grade from the two tables, then you need to map them to the corresponding objects.

Create a new VO that contains the objects you want to query and map them one by one.

There is no difference between a multi-table query and a single-table query, except that you need to include the result fields of multiple tables at the same time when an object receives the return value. Everything else makes no difference. mybatis itself does not interpret whether it is a multi-table query.

I don't understand what it means.,Mybatis multi-table query and single-table query is different?,Isn't it all written in the DAO as an excuse.,The methods inside and the id of the SQL statement in XML correspond to it?,Multi-table query can be written directly in SQL.。