Previous programmer left me with "beautiful" piece of code and he kind of forgot to apply something to it. There is a query which selects several items from several tables.
6 Items can be chosen. It means 6 tables can be chosen, however there can be more tables - even 20 of them. I need to get that list of tables from processed query.
Indeed, is it possible at all? Is there any command to get a list of tables the query used?
I'm unsure if I am understanding the question correctly but it may be worth 'explaining' the query and seeing which tables are being used as below.
EXPLAIN SELECT * FROM table1 JOIN
table2 USING (id)
Related
I have a quite complex view with two queries (a view in a view), one select users with related data and another one select orders with related data. Both of them have some filters, but now I have an issue and I am looking for proper and just decent solution, with good performance because I have a lot of data and relationships in the queries.
Assume I have:
Query 1 - Select user data, some left joins to other tables, conditions depends on provided parameters.
Query 2 - Select orders depends on users from Query 1, many joins, conditions depends on parameters.
I display data from two queries in one view, users, their data, orders, and some orders data and now I want to implement pager, but it has to work and display proper number of users depends on filters form Query 1 and Query 2. So there is an issue that I can't really limit from any query cuz another one has filters as well so maybe those users maybe aren't really selected to display depends on other query filters.
So I guess there are two ways, one is to put those queries in loop and collect data until I get proper number of results depends on query.
Another way is to merge those two queries into one, but there an issue that I get many rows per user, so I can't set any page limit and get results only for specific number of users, like for example 30. Because results will be like user 1 => order 1, user 1 => order 2, so is there any way to get specific number of unique results depends on user id or something.
Let me know if you have any questions.
Sample data will make more sense. I am unable to understand the whole requirement here in your question. will you be able to create some sample data and share with us ? if you are dealing with a lot of data, avoid loops as that will just make performance worse.
I am using sql join for refining some result but i am not getting my result as expected.
My query was
select order_detail.*, product.img1, product.added_by,
product.fa,product.name, commition.commission
from order_detail,product,commition
where order_detail.order_id='ODN314746M38'
and product.id=order_detail.product_id
and product.added_by='1'
and commition.subcat=product.subcat
As a result i should get only the one row but I am getting 2 rows with same result
My actual data table is
But I am getting 2 rows back as a result like
why this is happening?
Can anyone help me out?
There must be 2 rows in comition with a matching subcat. Add comition.id to the SELECT list to see this.
You will need to refine the relationship with the comition table so you only return the appropriate row for this order. Without knowing more about your table structure I can't be more specific.
It's also helpful to use ANSI JOIN as in the other answer. This makes it easier to see how all the tables relate to each other, and often makes it clearer where you're missing a condition.
Something really bugs me and im not sure what is the "correct" approach.
If i make a select to get contacts from my database there are a decent amount of joins involved.
It will look something like this (around 60-70 columns):
SELECT *
FROM contacts
LEFT JOIN company
LEFT JOIN person
LEFT JOIN address
LEFT JOIN person_communication
LEFT JOIN company_communication
LEFT JOIN categories
LEFT JOIN notes
company and person are 1:1 cardinality so its straight forward.
But "address", "communication" and "categories" are 1:n cardinality.
So depending on the amount of rows in the 1:n tables i will get a lot of "double" rows (I don't know whats the real term for that, the rows are not double i know that the address or phone number etc is different). For myself as a contact, a fairly filled contact, i get 85 rows back.
How do you guys work with that?
In my PHP application i always wrote some "Data-Mapper" where the array key was the "contact.ID aka primary" and then checked if it exists and then pushed the additional data into it. Also PHP is not really type strict what makes it easy.
Now I'm learning GO(golang) and i thought screw that LOOOONG select and data mapping just write selects for all the 1:n.... yeah no, not enough connections to load a table full of contacts. I know that i can increase the connections but the error seems to imply that this would be the wrong way.
I use the following driver: https://github.com/go-sql-driver/mysql
I also tried GROUP_CONCAT but then i running in trouble parsing it back.
Do i have to do my mapping approach again or is there some nice solution out there? I found it quite dirty at points tho?
The solution is simple: you need to execute more than one query!
The cause of all the "duplicate" rows is that you're generating a result called a Cartesian product. You are trying to join to several tables with 1:n relationships, but each of these has no relationship to the other, so there's no join condition restricting them with respect to each other.
Therefore you get a result with every combination of all the 1:n relationships. If you have 3 matches in address, 5 matches in communication, and 5 matches in categories, you'd get 3x5x5 = 75 rows.
So you need to run a separate SQL query for each of your 1:n relationships. Don't be afraid—MySQL can handle a few queries. You need them.
What methods are recommended for Selecting multiple columns within a nested subquery? It's been a while since I've coded any queries and I'm having some difficulty wrapping my head around this. The specific challenge is on Line 2 of the code below. The IN operand doesn't quite work here (see error message below), and I'm not sure if it's simply a matter of the syntax I'm using, and/or there is a much better way to go about this (i.e. using the HAVING operand or a JOIN statement)
SELECT * FROM Rules WHERE Rules.LNRule_id
IN(SELECT LNRule_id1,LNRule_id2,LNRule_id3,LNRule_id4 FROM Silhouette
WHERE Silhouette.Silhouette_Skirt=(SELECT Silhouette_Skirt FROM Style
WHERE Style.Style_Skirt='$Style_Skirt')
)
The purpose of this query is to SELECT all the relevant rows in table Rules for a particular value in table Style (i.e. $Style_Skirt), which it does by matching it to one of several factors - in this case the garment's Silhouette. What I am therefore trying to do in this portion of the query is SELECT all rows in table Rules who's ID (LNRule_id) matches values in any of the specified columns in table Silhouette
(SELECT LNRule_id1,LNRule_id2,LNRule_id3,LNRule_id4 FROM Silhouette WHERE Silhouette.Silhouette_Skirt=(...))
Edit
There is a many-to-many relationship (each Silhouette has several applicable Rules, and each Rule can apply to several Silhouettes). All the rules reside in the table 'Rules' (one per row), and each rule has an id ('LNRule_id'). The table 'Silhouette' has columns which tell it which rows need to be called from 'Rules' by 'LNRule_id' (LNRule_id1,2,3,4 indicate which Rules should be called, and store the values of the id's for the relevant rows in table 'Rules')
The error message currently being generated by the IN Operand is:
SQLSTATE[21000]: Cardinality violation: 1241 Operand should contain 1
column(s)
I think you want this query
SELECT * FROM Rules r
JOIN Silhouette s
(ON r.LNRule_id=s.LNRule_id1
OR r.LNRule_id=s.LNRule_id2
OR r.LNRule_id=s.LNRule_id3
OR r.LNRule_id=s.LNRule_id4)
JOIN Style st
ON s.Silhouette_Skirt=st.Silhouette_Skirt
WHERE st.Silhouette_Skirt = '$Style_Skirt'
mysql is complaining that on one side of IN you have a single column, and on the other side you have a multi-column rowset. In order for the IN operator to work, the rowset on the right side of IN must have the exact same number of columns as the left side; in this case, one column.
What you are trying to accomplish could perhaps be achieved if you did something like WHERE LNRule_id IN( SELECT LNRule_Id1 ...) OR LNRule_id IN( SELECT LNRule_Id2 ...) OR ... OR ... but the resulting query would be a monstrosity, and its performance would be horrendous. There may be other ways to go about it too, but anything you try will probably be similarly atrocious.
I do not have enough information to be absolutely sure about what I am saying, but it seems to me that the reason why you have this problem is that your database schema is not normalized. Generally, whenever you see a table with a group of columns having names that all begin with the same prefix and end with a number, it means that someone, somewhere, did not normalize their data.
To address the edit in your question, what you have implemented might conceptually be a many to many relationship, but as far as relational databases are concerned, (you know, the science, the theory, the established practices, the approaches necessary to get things to actually work,) it is definitely not a many to many relationship. Many to many relationships are most certainly not implemented with column1, column2, column3, ... columnN. To be sure that I am not making this stuff up, you can read what others say about many to many relationships here:
Many-to-many relations in RDBMS databases
So, my suggestion, if I correctly understand what is going on, would be to introduce a new table, called SilhouetteRules, which contains two columns, silhouette_id and rule_id. This table will implement a many-to-many relationship between silhouettes and your rules. Then of course you get rid of all the rule1, rule2, rule3, etc. columns from Silhouette.
Once you have done that, you can obtain all silhouettes and all rules associated with them using a query like this:
SELECT * FROM Silhouette
LEFT JOIN SilhouetteRules ON
Silhouette.id = SilhouetteRules.silhouette_id
LEFT JOIN Rules ON
SilhouetteRules.rule_id = Rules.id
The above query will yield multiple rows for each silhouette, where the silhouette fields will be identical from row to row, and only the rule fields will differ. Do not be surprised by this, that's how relational databases work.
Given a given_silhouette_id, you can retrieve all rules associated with it using a query like this:
SELECT * FROM Rules
LEFT JOIN SilhouetteRules ON
Rules.id = SilhouetteRules.rule_id
WHERE
SilhouetteRules.silhouette_id = given_silhouette_id
So, you are going to be using this query as a subquery in queries like the one in the question.
Now, regarding the query in the question, I am unable to tell you exactly how you would need to modify it to get it to work with the normalization that I proposed, because I cannot make sense of it. You see, even if you fix the problem that you currently have with SELECT * FROM table WHERE single-column IN multi-column-rowset, there is another problem further down: the WHERE Silhouette.Silhouette_Skirt=(SELECT ... part would not work either, because you cannot compare the value of a column against the result of a select statement. So, I do not know what you are trying to do there. Hopefully, once you normalize your schema and fix the first problem with your query, then the solution to the second problem will become obvious, or you can ask another question on stackoverflow.
P.S. did Mihai's answer work?
I basically have results grid and i have a drop down menu on my application which filters the 'Carrier' column. But when selecting a certain carrier I want all the rows returned that have the same dr_id as the Carrier which has been selected.
For example if you look at the picture attached it shows my results grid. If I filter by carrier 'ACE CALL LTD_UK' then I want rows 27, 28, 29 and 30 returned because the dr_id is the same.
Thanks
I don't have a complete solution for you as I don't know exactly what you database schema is (and it is a large stored procedure!). However I do have some suggestions/comments which you might find helpful:
I assume that the stored procedure will currently be returning a single row when they filter is set to 'ACE CALL LTD_UK', if not then this might not be relevant!
What I would do in this case would be to take you SELECT statement and put the results into a CTE, temporary table or nested query. (I'm not sure what SQL DBMS your using, looks like MSSQL, but you also have a MySQL tag for you post).
Once I have those results I would then use a LEFT JOIN from the dr_id in the temp table back to the drm table on the same column. From here you will again need to join to other tables where the data is not distinct, for example the Carrier table, then select the columns that you require.
You could do all this in the existing SELECT statement, however you will have to start joining on tables multiple times or use nested queries, which would get very messy. However the main reason why I chose the solution I have posted is because I don't know the stored procedure well enough and therefore I chose the safest solution.
If you want an example of what I mean, I will try and provide one.