How to map two variables from SQL Query - MyBatis - sql

Question

So, I have this select query below, which joins two tables and retrieves a String:
<select id =“getAppVerByConfId” parameterType=“java.lang.String” resultType=“java.lang.String”>
SELECT t.app_ver
FROM
application a
JOIN transaction t on t.txn_id = a.txn_id
WHERE
a.confirmation_id = #{0}
</select>
and then I used that as a template to write a 2nd query, which is nearly identical, but just retrieves a different parameter from the table.
<select id =“getStepNameByConfId” parameterType=“java.lang.String” resultType=“java.lang.String”>
SELECT t.step_name
FROM
application a
JOIN transaction t on t.txn_id = a.txn_id
WHERE
a.confirmation_id = #{0}
Both of these work fine on their own, and they're used at the same point in the program. But there's got to be a better way than this surely? I should be able to make the query once, and then map the results to what I want, correct? Should I make a resultset, and then be able to pull them out? Maybe as a HashMap and I can retrieve the values by keys? Is this a situation where I can USE the AS operator? i.e. "SELECT t.app_ver AS appVersion"? My thinking is that's for passing variables into the query, though, and not for getting them out?
If there's any thoughts on this I would love to hear them. I'm basically trying to combine these into one query, and I need to be able to retrieve the right value and not assign app_ver to step_name or vice versa.
Cheers

Answer 1

As you say it's not a bad idea use alias (t.app_ver as appVersion) in your select but it is just the name of the column which will be mapped. So in the case you use alias as next t.app_ver as appVersion, t.step_name as stepName your column names will be appVersion and stepName.
Then, to map your result you have multiple choices the idea to map it in a map structure is not a bad idea and it's easy, you just need to put your result type as hashmap, it will be something like that (and it's not need any Resultmap):
Hashmap
(Example in offical page)
<select id="selectPerson" parameterType="int" resultType="hashmap">
SELECT * FROM PERSON WHERE ID = #{id}
</select>
The column will be the key and the row values the value in the map.
keyed by column names mapped to row values
So to get your values you will need to use the column name as key in the map:
String appVersionValue = map.get("appVersion");
Resultmap
Other way it to create a class with the properties you need to map and then create your resultmap.
A resultmap is defined as next:
resultMap – The most complicated and powerful element that describes
how to load your objects from the database result sets.
Your class would be like:
public class Application{
private String appVersion;
private String stepName;
//.... getters and setters
}
And your result map would map the column name with the class properties specifying the type with the class created for this (In this case is Application):
<resultMap id="applicationResultMap" type="Application">
<result property="appVersion" column="appVersion"/>
<result property="stepName" column="stepName"/>
</resultMap>
(Be careful, because in this example the columns and properties are called equal, there are cases where the column is called app_version and the property appVersion for example so there would be <result property="appVersion" column="app_version"/>
Finally in your select you specify to use this resultmap:
<select id="selectMethodName" resultMap="applicationResultMap">
select t.app_ver as appVersion, t.step_name as stepName
from your_table
</select>