获取多个选择查询结果并为单行创建差异列

Question

I have a simple select query in db2 that works but I'm trying to find a way to turn result records into columns for one result record. I have:

SELECT item_id, t2.item_title, t2.item_data
FROM item t1 
INNER JOIN title t2
ON  t1.item_id = t2.item_id

This gives

item_id | item_title  |   item_data
-----------------------------------
123         Main          Test Title
123         Sub           Subtitle

Is there a way I can alter the select to get the data for each title type as a column and only return one row?

so I want to get main and sub as their own column and return:

item_id | main_data  |   sub_data
-----------------------------------
123      Test Title      Subtitle

Answer 1

This is a pivot operation, and a cross-platform way to achieve it looks like this:

Step 1: break the data into its own columns for main and sub:

SELECT 
  t1.item_id, 
  CASE WHEN t2.item_title = 'Main' THEN t2.item_data END as main_title,
  CASE WHEN t2.item_title = 'Sub' THEN t2.item_data END as sub_title
FROM item t1 
INNER JOIN title t2
ON  t1.item_id = t2.item_id

Step 2: squish the results down to one row, getting rid of the nulls using group/max

SELECT 
  t1.item_id, 
  MAX(CASE WHEN t2.item_title = 'Main' THEN t2.item_data END) as main_title,
  MAX(CASE WHEN t2.item_title = 'Sub' THEN t2.item_data END) as sub_title
FROM item t1 
INNER JOIN title t2
ON  t1.item_id = t2.item_id
GROUP BY t1.item_id

Answer 2

This is a pivot operation,you can try this WITH Opening AS( SELECT item_id, t2.item_title, t2.item_data FROM item t1 INNER JOIN title t2 ON
t1.item_id = t2.item_id) SELECT item_id,Main ,Sub PIVOT( item_data FOR item_title IN([Main],[Sub]) ) as PivotTable FROM Opening;