I want to do self-join I have written a code as well but its throwing an error and it seems that there is some issue with an alias.
Apart from this it would also be helpful for me if someone let me know any best site where I can learn query in MS Access. I searched a number of places everywhere, it is showing through UI Interface but I want to learn query in MS Access.
(SELECT distinct itemname,vendorname,price,count(*)
from vendor_Details1
group by itemname,vendorname,price
order by vendorname) A
inner join
(SELECT distinct itemname,vendorname,price,count(*)
from vendor_Details1
group by itemname,vendorname,price
order by vendorname) B
on A.vendorname=B.vendorname
It is entirely unclear what you are trying to do. However, "join" is an operator in the FROM clause that operates on two tables, views, or subqueries.
The structure of a self-join looks like:
select . . . -- list of columns here
from t as t1 inner join -- you need aliases for the table so you can distinguish the references
t as t2
on t1.? = t2.? -- the join condition goes here
Your query doesn't even have a select.
Related
I have a few questions about the select statement.
First of all, I have normalized 15 tables for this select query.
The problem is invisible because there is not much data right now.
However, since I try to process many tables in one select query, it seems to cause problems later.
So I want to add a few more select statements to divide the tables to search, but I want to know how different it is from doing it at once.
Secondly, if I use join, I will use outer join. If I join multiple tables with outer join, I'm not sure how to use left outer join and right outer join.
The currently created select query refers to 8 tables and one join is linked.
That is, the remaining rest of the tables have obtained data in subqueries and the remaining eight tables are likely to use join.
I would appreciate it if you could let me know the direction of the multiple outer joins.
Let me briefly show you some of the current select queries.
select
a.cal1,a.cal2,a.cal3,...,
(select b.cal1 from b
where a.cal4=b.cal2)
as "bcals",
(select c.cal1 from c
where a.cal5=c.cal2)
as "ccals",
....,
(select e.cal1 from e
where a.caln=e.cal2)
as "ecals",
(select sum(extract(year from age(f.endday,f.startday))
from f
where e.cal1=a.cal1)
as "fcals",
g.cal1,g.cal2,g.cal3,...,
(select h.cal1 from h
where g.cal4=h.cal2)
as "hcals"
from a left outer join g on a.cal1=g.cal5
where a.cal1=?;
Result:
a.cal1|a.cal2|a.cal3|...|hcals
var1 |var2 |var3 |...|varn
After this, I wonder how to join the rest of the tables.
To sum up
If there are many tables that need to be included in a select query statement, what is the difference between performance and performance when this complex query is divided into multiple select statements?
If we write inside a select statement, how should outer join be?
Is there a problem with the query?
Actually your code is correct, but it looks very complex. People will find it difficult to understand it. Using joins you can minimize the lines of code and also make it more readable.
SELECT
TBL1.AMOUNT T1,
TBL2.AMOUNT T2,
TBL3.AMOUNT T3
FROM TBL1
LEFT JOIN TBL2 ON TBL2.ID = TBL1.ID
LEFT JOIN TBL3 ON TBL3.ID = TBL1.ID
In the above code , there are three tables, and two joins. One can easily understand and debug/make changes. Please try this for your code.
I have a very simple query that I am trying to execute:
select *
from submissions
inner join (
select *
from hackers
inner join challenges
on hackers.hacker_id = challenges.hacker_id
) dtable
on submissions.challenge_id = dtable.challenge_id
and submissions.hacker_id = dtable.hacker_id;
Oracle rejects it with:
ORA-00904: "DTABLE"."HACKER_ID": invalid identifier.
I have kept the alias dtable visible by keeping it outside the brackets. Why is Oracle rejecting my query?
SELECT * in your sub-query is a problem.
You have the same column name in both tables that are being joined. This means that you're trying to create an inline-view called dtable where at least two columns have the same name (in this case both tables have a hacker_id column and your use of * is essentially saying "use them both".). You can't do that.
You're going to need SELECT hackers.a, hackers.b, challenges.x, challenges.y, etc, etc in your sub-query. By being explicit in this way you can ensure that no two columns have the same name.
An alternative could be SELECT hackers.*, challenges.a AS c_a, challenges.b AS c_b, etc.
Either way you are being explicit about which fields to pick up, what their positions and names are, etc. The end result being that you can then avoid columns having the same name as other columns.
You do not need subqueries for this. Your query is not actually "simple". The simple form looks more like this:
select . . .
from submissions s join
hackers h
on s.hacker_id = h.hacker_id join
challenges c
on s.challenge_id = c.challenge_id;
Note that I removed the condition between challenge and hackers on hacker_id. That extra join condition doesn't really make sense to me (although it might make sense if you provided sample data).
As others have said: The sub-select selects two different columns hacker_id from two different tables. This confuses Oracle.
But there is no need for a sub-select
select * from
submissions
inner join challenges
on submissions.challenge_id = challenges.challenge_id
inner join hackers
on submissions.hacker_id = hackers.hacker_id;
In http://www.w3schools.com/sql/sql_alias.asp, it mentions using alias to do the following query,
SELECT
Orders.OrderID, Orders.OrderDate, Customers.CustomerName
FROM
Customers, Orders
WHERE
Customers.CustomerName = "Around the Horn"
AND Customers.CustomerID = Orders.CustomerID;
This confuses me with the usage of JOIN. Isn't this kind of query joining the columns from two tables? What are the differences between this kind of query and JOIN?
JOIN and alias are two differnt concept .. the alias is for create a substitutive name (shorter usually) for a more easy object reference and for a more easy read .. so you can have column name alias or table name alias eg:
select a.col1
from my_table as a
a is an alias for the table my_table
or
select a.col1 as c1
from my_table as a
where c1 is an alias for col1
JOIN are for build relation between table
The Join can be implict or explict
In your code you are using implici join and the condition between the tables that keep the relation is based on where clause
but you could use a more espressive way using explict join
SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName
FROM Customers
INNER JOIN Orders on Customers.CustomerID=Orders.CustomerID;
WHERE Customers.CustomerName="Around the Horn"
Both the same, there is no difference.
There are differences only in readability.
In my opinion, PLSQL(ORACLE) developer choosing alias when writing queries then TSQL(SQL Server) developer choosing by Join
The answer to your question is there is no difference between your query and an inner join but many times, your write queries between tables where the relationships are not explicitly defined or a table may not require a relationship. In those cases, you would use a left join to return data from a first table and zero to many items from the table on the right. Using your format, makes that a lot more difficult to write and read. As for table Aliases, when writing self joins for example, you will need to use them so understanding them is essential.
I have five results to retrieve from a table and I want to write a store procedure that will return all desired rows.
I can write the query like that temporarily:
Select * from Table where Id = 1 OR Id = 2 or Id = 3
I supposed I need to receive a list of Ids to split, but how do I write the WHERE clause?
So, if you're just trying to learn SQL, this is a short and good example to get to know the IN operator. The following query has the same result as your attempt.
SELECT *
FROM TABLE
WHERE ID IN (SELECT ID FROM TALBE2)
This translates into what is your attempt. And judging by your attempt, this might be the simplest version for you to understand. Although, in the future I would recommend using a JOIN.
A JOIN has the same functionality as the previous code, but will be a better alternative. If you are curious to read more about JOINs, here are a few links from the most important sources
Joins - wikipedia
and also a visual representation of how different types of JOIN work
Another way to do it. The inner join will only include rows from T1 that match up with a row from T2 via the Id field.
select T1.* from T1 inner join T2 on T1.Id = T2.Id
In practice, inner joins are usually preferable to subqueries for performance reasons.
Can you guys help me please in understanding the SQL specifications on join. I don't understand it. I kept getting errors called Unknown column list on on clause.
I got this error over my SQL syntax, I almost rubbed it in my face I just can't understand why it is not working, I have read some article regarding that it is because of precedence etc but I am really confused on what I have done wrong here.
select product.name , product.price from product inner join product_category on
(product_category.product_no = product.product_no ) where product_category.sub_category =
"COFFIN";
I know this question have been ask a hudred and million times here, but the ones I saw are complicated nowhere close in this very basic sql syntax.
THanks for helping me out.
EDIT:
I just had realize that I have product_category not a direct child to my product table so
I just have typed
select * from product
join specifications
join product_category on ( specifications.product_no = product_category.product_no);
But this still gave me an error, unknown column product_category.
I've read and followed some instruction similarly to this sites:
MYSQL unknown clause join column in next join
Unknown column {0} in on clause
MySQL "Unknown Column in On Clause"
I am really frustrated. I really can't get it to work.
For each new table you join in your query, each table must have at least one ON clause. It's hard to know exactly what you're trying to do without knowing the schema (table names, columns, etc), but here's an example
select *
from product p
join specifications s
on p.product_no = s.product_no
join product_category pc
on pc.spec_no = p.spec_no
Check out this link on table aliases as well. Gives a good example on joins + really useful info on how to increase the readability of your SQL
http://msdn.microsoft.com/en-us/library/ms187455(v=sql.90).aspx
I found this article useful as well as it visually displays the different types of joins
http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
You are missing the part where you specify the joining conditions between product and specifcations.
select * from product
join specifications YOU NEED SOMETHING HERE
join product_category on etc
I modified the SQL syntax to look like this, I have overlook a key that connects product_category onto specification so I made necessary link and it worked!!!
SELECT *
FROM product
JOIN specifications ON ( product.product_no = specifications.product_no )
JOIN product_category ON ( specifications.spec_no = product_category.spec_no )
WHERE product_category.sub_category = "COFFIN"
LIMIT 0 , 30
Also thanks for the heads up on missing joining condition on specifications. Heck this carelessness cost so much time.
Thank you so much!
The default join type is an inner join. So if you write join, the database reads inner join, and insist that you include an on clause.
If you'd like to join without a condition, specify the cross join explicitly:
select *
from product p
cross join
specifications s
inner join
product_category pc
on pc.product_no = p.product_no
left join
some_other_table sot
on 1=1
The last join, with the on 1=1 condition, is another way to do a cross join. It's subtly different in that it will return rows from the left table even if the right table is empty.
Example at SQL Fiddle.