Is it possible to write a statement in sqlite.swift that will generate the equivalent sql:
SELECT foods.name, food_types.name FROM foods, food_types
WHERE foods.type_id=food_types.id LIMIT 10;
I can't figure out how to query from multiple Table objects at once.
Thanks!
Your original query passes two tables to the FROM clause, creating an implicit join. SQLite.swift's query builder language currently only supports explicit joins.
Check out the documentation under Joining Other Tables for more information on joining tables.
In your case:
let foods = Table("foods")
let food_types = Table("food_types")
let name = Expression<String>("name")
let id = Expression<Int64>("id")
let type_id = Expression<Int64>("type_id")
let query = foods
.select(foods[name], food_types[name])
.join(food_types, on: foods[type_id] == food_types[id])
.limit(10)
I figured it out. The foreign key is a column for all the tables I'm trying to join, but there are foreign key members that are not common among all the tables so I believe sql then generates a a cross join vs an inner join... That leads to all the extra rows in the query. I confirmed this by using the sql that sqlite.swift generates on the db directly.
Related
I am trying to use FULLTEXT search as a preliminary filter before fetching data from another table. Consecutive JOINs follow to further refine the query and to mix-and-match rows (in reality there are up to 6 JOINs of the main table).
The first "filter" returns the IDs of the rows that are useful, so after joining I have a subset to continue with. My issue is performance, however, and my lack of understanding of how the SQL query is executed in SQLite.
SELECT *
FROM mytbl AS t1
JOIN
(SELECT someid
FROM myftstbl
WHERE
myftstbl MATCH 'MATCHME') AS prior
ON
t1.someid = prior.someid
AND t1.othercol = 'somevalue'
JOIN mytbl AS t2
ON
t2.someid = prior.someid
/* Or is this faster? t2.someid = t1.someid */
My thought process for the query above is that first, we retrieve the matched IDs from the myftstbl table and use those to JOIN on the main table t1 to get a sub-selection. Then we again JOIN a duplicate of the main table as t2. The part that I am unsure of is which approach would be faster: using the IDs from the matches, or from t2?
In other words: when I refer to t1.someid inside the second JOIN, does that contain only the someids after the first JOIN (so only those at the intersection of prior and those for which t1.othercol = 'somevalue) OR does it contain all the original someids of the whole original table?
You can assume that all columns are indexed. In fact, when I use one or the other approach, I find with EXPLAIN QUERY PLAN that different indices are being used for each query. So there must be a difference between the two.
The query should be simplified to
SELECT *
FROM mytbl AS t1
JOIN myftstbl USING (someid) -- or ON t1.someid = myftstbl.someid
JOIN mytbl AS t2 USING (someid) -- or ON t1.someid = t2.someid
WHERE myftstbl.{???} MATCH 'MATCHME' -- replace {???} with correct column name
AND t1.othercol = 'somevalue'
PS. The query logic is not clear for me, so it is saved as-is.
I need to update a column in one table with the results from a select sub-query (and they should ultimately be different). But When I do this, I get the 'ORA-01427: single row sub-query returns more than one row query' error.
Can you please take a look and see what it is that I am overlooking? (I could just be overlooking something simple for all I know)
UPDATE AIRMODEL_NETWORK_SUMMARY ans
SET ANS.NBR_RETURNS = (
SELECT SUM(RQ.RETURN_QTY)
FROM RETURN_QTY RQ JOIN AIRMODEL_NETWORK_SUMMARY ANS ON RQ.LOC_ID = ANS.LOC_ID
WHERE RQ.FSCL_YR_NUM = ans.FSCL_YR_NUM
AND RQ.FSCL_WK_IN_YR_NUM =
ans.FSCL_WK_IN_YR_NUM
GROUP BY ANS.LOC_ID,
ans.FSCL_WK_IN_YR_NUM,
ANS.FSCL_YR_NUM
);
I think that your inner query is not well correlated to the table that you're trying to update. Please look here Oracle SQL: Update a table with data from another table. You should add some kind of a where condition that ties the rows you're trying to update with the values calculated by the inner statement.
I have a collection of IDs in memory, and I would like to fetch only rows from a DB matching those IDs.
In SQL, I could either write a query like SELECT * FROM mytable WHERE id IN (1,3,5,10) or do a join between tables.
My problem is that EF can't build a query where I join my EF-data with my local array or list.
(I'm using EF4.1, but I'm guessing the problem/solution would be similar in older versions, as well as with Linq-to-SQL.)
You can use Contains() with your ID collection myIDs to generate the equivalent WHERE id IN .. query:
var results = context.mytable.Where(x => myIds.Contains(x.Id));
My question might seem weird. But I need to know whether we can identify and seggregate the rows fetched from particular table in a query, which is formed by joining tables.
Please refer the below query if I am you are not clear with my question.
My requirement is to know the rows fetched from table 'rel' alone in the query.
SELECT rel.*
FROM rel, C
WHERE rel.col1 = C.col1
AND C.col1 NOT IN (SELECT R.col1 FROM R WHERE R.col2 = 'MN')
AND rel.col2= 'MN'
AND rel.col3= 'MN'
AND c.col2 ='MN';
Thanks,
Savitha
Since you are performing an INNER JOIN by using the comparison in the WHERE clause, all your records, by definition, are coming from the rel table.
I have two entities: Master and Details.
When I query them, the resulting query to database is:
SELECT [Extent2]."needed columns listed here", [Extent1]."needed columns listed here"
FROM (SELECT * [Details]."all columns listed here"...
FROM [dbo].[Details] AS [Details]) AS [Extent1]
LEFT OUTER JOIN [dbo].[Master] AS [Extent2] ON [Extent1].[key] = [Extent2].[key]
WHERE [Extent1].[filterColumn] = #p__linq__0
My question is: why not the filter is in the inner query? How can I get this query? I've tried a lot of EF and Linq expressions.
What I need is something like:
SELECT <anything needed>
FROM Master LEFT JOIN Details ON Master.key = Details.Key
WHERE filterColumn = #param
I'm having a full sequential scan in both tables, and in my production environment, I have milions of rows in each table.
Thanks a lot !!
Sometimes The entity Framework does not produce the best query. You can do a few of the following to optimize.
Modify the linq statement (test with
LINQPad)
Create a stored proc and map the stored proc to return an entity
Create a view that handles the join and map the view to a new
entity