Combining data from 2 tables in to 1 dynamic query - sql

Question

I have two tables:
table 1
id item itemType
-----------------------
1 book1 1
2 book2 1
3 laptop1 2
table 2
id itemId name value
------------------------------------------
1 1 author enid blyton
2 1 title five 1
3 2 author enid blyton
4 2 title five 2
5 3 cpu i7-940
6 3 ram 4 GB
7 3 vcard nvidia quadro
When I query with filter itemType = 1, the result should be:
query 1
id item author title
--------------------------------------------------------
1 book1 enid blyton five 1
2 book2 enid blyton five 2
and with filter itemType = 2
query 2
id item cpu ram vcard
----------------------------------------------
1 laptop1 i7-940 4 GB nvidia quadro
and without filter
query 3
id item author title cpu ram vcard
---------------------------------------------------------------------------
1 book1 enid blyton five 1
2 book2 enid blyton five 2
1 laptop1 i7-940 4 GB nvidia quadro
The reason I use table 2 is because the parameter of each itemType is created during the fly, so it is not possible to have a table like in query 3.
At this moment I can solve this in C# by rebuilding the table programmatically (using a lot of linq call). With a small size of table 1 (1K rows) and 2 (10K rows), the performance is good, but now the size of table 1 is already more than 100K rows and table 2 is more than 1M rows, and the performance is very low.
Is there any function using SQL query that can solve this problem?

Answer 1

Not exactly dynamic but if your name's are all known upfront, you can use PIVOT to retrieve your data.
PIVOT rotates a table-valued expression by turning the unique values
from one column in the expression into multiple columns in the output,
and performs aggregations where they are required on any remaining
column values that are wanted in the final output.
SQL Statement
SELECT t1.Id
, t1.item
, t2.author
, t2.title
, t2.cpu
, t2.ram
, t2.vcard
FROM table1 t1
INNER JOIN (
SELECT *
FROM (
SELECT itemId
, name
, value
FROM table2
) s
PIVOT (
MAX(Value)
FOR name IN (title, author, cpu, ram, vcard)
) p
) t2 ON t2.itemId = t1.Id
Test script
;WITH table1 (id, item, itemtype) AS (
SELECT 1, 'book1', 1
UNION ALL SELECT 2, 'book2', 1
UNION ALL SELECT 3, 'laptop1', 2
)
, table2 (id, itemId, name, value) AS (
SELECT 1, 1, 'author', 'enid blyton'
UNION ALL SELECT 2, 1, 'title', 'five 1'
UNION ALL SELECT 3, 2, 'author', 'enid blyton'
UNION ALL SELECT 4, 2, 'title', 'five 2'
UNION ALL SELECT 5, 3, 'cpu', 'i7 940'
UNION ALL SELECT 6, 3, 'ram', '4 GB'
UNION ALL SELECT 7, 3, 'vcard', 'nvidia quadro'
)
SELECT t1.Id
, t1.item
, t2.author
, t2.title
, t2.cpu
, t2.ram
, t2.vcard
FROM table1 t1
INNER JOIN (
SELECT *
FROM (
SELECT itemId
, name
, value
FROM table2
) s
PIVOT (
MAX(Value)
FOR name IN (title, author, cpu, ram, vcard)
) p
) t2 ON t2.itemId = t1.Id

Answer 2

I suggest running a query to return all possible names from table2 for the specified itemtype, like so:
select distinct name
from table2 t2
where exists (select null
from table1 t1
where t1.itemtype = #itemtype and
t1.id = t2.item_id)
In C#, concatenate the names into a single comma-separated string, then construct a new query string similar to Lieven's answer, like so:
SELECT t1.item
, t2.*
FROM table1 t1
INNER JOIN (SELECT *
FROM (SELECT itemId,
name,
value
FROM table2) s
PIVOT (MAX(Value)
FOR name IN (/*insert names string here*/)) p
) t2 ON t2.itemId = t1.Id
WHERE t1.itemtype = #itemtype;
(with the names string replacing the comment inside the brackets).
Incidentally, if possible, I suggest separating the names from Table 2 into a separate lookup table, like so:
name_table
----------
name_id
name
itemtype
- this would mean that the first query would only have to query a small lookup table rather than all of table 2; it could also be used for consistency in name values at data entry.