I never got my head around the "new" SQL join syntax, and therefore use the "old" join system, with the (+). I know it's about time I learned it - however I just find the old syntax a lot more intuitive, especially when working with multiple tables with multiple joins.
However I now have an operation which requires two outer joins on the same table. My code is:
SELECT
C.ID,
R.VALUE,
R.LOG_ID,
LOG.ACTION
FROM
C,
R,
LOG
WHERE
C.DELETED IS NULL
AND R.DELETED IS NULL
-- Two joins below
AND R.C_ID(+) = C.ID
AND R.LOG_ID(+) = LOG.ID
However this results in an error:
ORA-01417 - A table may be outer joined to at most one table.
Searching for this error I find that the solution is to use the new syntax For example this answer on SO:
Outer join between three tables causing Oracle ORA-01417 error
So I am aware that some may consider this question a duplicate as it technically already has an answer. However the "old" syntax posed in that question does not contain exactly the same number of tables and joins as I have here, and try as I might, I'm not sure how I would factor this in to my own code.
Is anyone able to assist? Thanks.
I think you want:
SELECT C.ID, R.VALUE, R.LOG_ID, LOG.ACTION
FROM C LEFT JOIN
R
ON R.C_ID = C.ID LEFT JOIN
LOG
ON R.LOG_ID = LOG.I
WHERE C.DELETED IS NULL AND
R.DELETED IS NULL;
The "new" (it is 25 years old) outer join syntax is actually very easy to follow, particularly for a simple example with just LEFT JOIN.
The idea is you want to keep all rows from one table (perhaps subject to filters in the WHERE clause). That is the first table. Then you use a chain of LEFT JOIN to bring in other tables.
All rows from the first table are in the result set. If there are matching rows in the other tables, then columns from those tables come from matching rows. If there are no matches, then the row from the first table is kept.
Related
I have a query that joins many related tables together and as a result, it returns duplicate rows for those items with multiple data against them.
I've searched for answers to this on stack and via google but all the results show things like using 'DISTINCT' or creating a subquery. I can't get any solution to work and I think the confusion I face is because of the many joins I have.
Can someone guide me on how to stop my results shows duplicates? Here is my query so far.
SELECT dbo.Vessel.VesselId,
dbo.Vessel.Name,
dbo.Capacity.DeckAreaM2,
dbo.Vessel.DPClassId,
dbo.Subsea.Accomodation,
dbo.Subsea.RovHangar,
dbo.Crane.SWL
FROM dbo.Vessel INNER JOIN
dbo.Capacity ON dbo.Vessel.VesselId = dbo.Capacity.VesselId LEFT OUTER JOIN
dbo.DeckEquipment ON dbo.Vessel.VesselId = dbo.DeckEquipment.VesselId LEFT OUTER JOIN
dbo.Crane ON dbo.DeckEquipment.DeckEquipmentId = dbo.Crane.DeckEquipmentId LEFT OUTER JOIN
dbo.Subsea ON dbo.Vessel.VesselId = dbo.Subsea.VesselId
First of all, does your query even work? There's no such thing as LEFT INNER JOIN ; you may have an INNER JOIN, a LEFT JOIN, or a LEFT OUTER JOIN, with the latter two being the same.
Second, I can understand your non-willingness to make an additional subquery, but why are you against the DISTINCT operator?
Third, if you use a GROUP BY and put there ONLY the fields you want, it will be equivelant to a DISTINCT operator and will return the results you need.
Last but not least, you need to show us what you are getting and what you want instead, if we are to be able to help you more.
Can we achieve desired results of joining tables by executing joins in whatever order? Suppose we want to left join two tables A and B (order AB). We can get the same results with right join of B and A (BA).
What about 3 tables ABC. Can we get whatever results by only changing order and joins types? For example A left join B inner join C. Can we get it with BAC order? What about if we have 4 or more tables?
Update.
The question Does the join order matter in SQL? is about inner join type. Agreed that then the order of join doesn't matter. The answer provided in that question does not answer my question whether it is possible to get desired results of joining tables with whatever original join types (join types here) by choosing whatever order of tables we like, and achieve this goal only by manipulating with join types.
In an inner join, the ordering of the tables in the join doesn't matter - the same rows will make up the result set regardless of the order they are in the join statement.
In either a left or right outer join, the order DOES matter. In A left join B, your result set will contain one row for every record in table A, irrespective of whether there is a matching row in table B. If there are non matching rows, this is likely to be a different result set to B left join A.
In a full outer join, the order again doesn't matter - rows will be produced for each row in each joined table no matter what their order.
Regarding A left join B vs B right join A - these will produce the same results. In simple cases with 2 tables, swapping the tables and changing the direction of the outer join will result in the same result set.
This will also apply to 3 or more tables if all of the outer joins are in the same direction - A left join B left join C will give the same set of results as C right join B right join A.
If you start mixing left and right joins, then you will need to start being more careful. There will almost always be a way to make an equivalent query with re-ordered tables, but at that point sub-queries or bracketing off expressions might be the best way to clarify what you are doing.
As another commenter states, using whatever makes your purpose most clear is usually the best option. The ordering of the tables in your query should make little or no difference performance wise, as the query optimiser should work this out (although the only way to be sure of this would be to check the execution plans for each option with your own queries and data).
I'm studying SQL for a database exam and the way I've seen SQL is they way it looks on this page:
http://en.wikipedia.org/wiki/Star_schema
IE join written the way Join <table name> On <table attribute> and then the join condition for the selection. My course book and my exercises given to me from the academic institution however, use only natural join in their examples. So when is it right to use natural join? Should natural join be used if the query can also be written using JOIN .. ON ?
Thanks for any answer or comment
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
IMO, the JOIN ON syntax is much more readable and maintainable than the natural join syntax. Natural joins is a leftover of some old standards, and I try to avoid it like the plague.
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.
Different Joins
* JOIN: Return rows when there is at least one match in both tables
* LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
* RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
* FULL JOIN: Return rows when there is a match in one of the tables
INNER JOIN
http://www.w3schools.com/sql/sql_join_inner.asp
FULL JOIN
http://www.w3schools.com/sql/sql_join_full.asp
A natural join is said to be an abomination because it does not allow qualifying key columns, which makes it confusing. Because you never know which "common" columns are being used to join two tables simply by looking at the sql statement.
A NATURAL JOIN matches on any shared column names between the tables, whereas an INNER JOIN only matches on the given ON condition.
The joins often interchangeable and usually produce the same results. However, there are some important considerations to make:
If a NATURAL JOIN finds no matching columns, it returns the cross
product. This could produce disastrous results if the schema is
modified. On the other hand, an INNER JOIN will return a 'column does
not exist' error. This is much more fault tolerant.
An INNER JOIN self-documents with its ON clause, resulting in a
clearer query that describes the table schema to the reader.
An INNER JOIN results in a maintainable and reusable query in
which the column names can be swapped in and out with changes in the
use case or table schema.
The programmer can notice column name mis-matches (e.g. item_ID vs itemID) sooner if they are forced to define the ON predicate.
Otherwise, a NATURAL JOIN is still a good choice for a quick, ad-hoc query.
I have a query which works, goes like this:
Select
count(InsuranceOrderLine.AntallPotensiale) as potensiale,
COUNT(InsuranceOrderLine.AntallSolgt) as Solgt,
InsuranceProduct.Name,
InsuranceProductCategory.Name as Kategori
From
InsuranceOrderLine, InsuranceProduct, InsuranceProductCategory
where
InsuranceOrderLine.FKInsuranceProductId = InsuranceProduct.InsuranceProductID
and InsuranceProduct.FKInsuranceProductCategory = InsuranceProductCategory.InsuranceProductCategoryID
Group by
InsuranceProduct.name, InsuranceProductCategory.Name
This query over returns what I need, but when I try to add more table (InsuranceOrder) to be able to get the regardingUser column, then all the count values are way high.
Select
count(InsuranceOrderLine.AntallPotensiale) as Potensiale,
COUNT(InsuranceOrderLine.AntallSolgt) as Solgt,
InsuranceProduct.Name,
InsuranceProductCategory.Name as Kategori,
RegardingUser
From
InsuranceOrderLine, InsuranceProduct, InsuranceProductCategory, InsuranceSalesLead
where
InsuranceOrderLine.FKInsuranceProductId = InsuranceProduct.InsuranceProductID
and InsuranceProduct.FKInsuranceProductCategory = InsuranceProductCategory.InsuranceProductCategoryID
Group by
InsuranceProduct.name, InsuranceProductCategory.Name,RegardingUser
Thanks in advance
You're adding one more table to your FROM statement, but you don't specify any JOIN condition for that table - so your previous result set will do a FULL OUTER JOIN (cartesian product) with your new table! Of course you'll get duplication of data....
That's one of the reasons that I'm recommending never to use that old, legacy style JOIN - do not simply list a comma-separated bunch of tables in your FROM statement.
Always use the new ANSI standard JOIN syntax with INNER JOIN, LEFT OUTER JOIN and so on:
SELECT
count(iol.AntallPotensiale) as Potensiale,
COUNT(iol.AntallSolgt) as Solgt,
ip.Name,
ipc.Name as Kategori,
isl.RegardingUser
FROM
dbo.InsuranceOrderLine iol
INNER JOIN
dbo.InsuranceProduct ip ON iol.FKInsuranceProductId = ip.InsuranceProductID
INNER JOIN
dbo.InsuranceProductCategory ipc ON ip.FKInsuranceProductCategory = ipc.InsuranceProductCategoryID
INNER JOIN
dbo.InsuranceSalesLead isl ON ???????? -- JOIN condition missing here !!
When you do this, you first of all see right away that you're missing a JOIN condition here - how is this new table InsuranceSalesLead linked to any of the other tables already used in this SQL statement??
And secondly, your intent is much clearer, since the JOIN conditions linking the tables are where they belong - right with the JOIN - and don't clutter up your WHERE clauses ...
It looks like you added the table join which slightly multiplies count of rows - make sure, that you properly joining the table. And be careful with aggregate functions over several joined tables - joins very often lead to duplicates
There must be something I'm missing here. I have this nice, pretty Oracle SQL statement in Toad that gives me back a list of all active personnel with the IDs that I want:
SELECT PERSONNEL.PERSON_ID,
PERSONNEL.NAME_LAST_KEY,
PERSONNEL.NAME_FIRST_KEY,
PA_EID.ALIAS EID,
PA_IDTWO.ALIAS IDTWO,
PA_LIC.ALIAS LICENSENO
FROM PERSONNEL
LEFT JOIN PERSONNEL_ALIAS PA_EID
ON PERSONNEL.PERSON_ID = PA_EID.PERSON_ID
AND PA_EID.PERSONNEL_ALIAS_TYPE_CD = 1086
AND PA_EID.ALIAS_POOL_CD = 3796547
AND PERSONNEL.ACTIVE_IND = 1
LEFT JOIN PERSONNEL_ALIAS PA_IDTWO
ON PERSONNEL.PERSON_ID = PA_IDTWO.PERSON_ID
AND PA_IDTWO.PERSONNEL_ALIAS_TYPE_CD = 3839085
AND PA_IDTWO.ACTIVE_IND = 1
LEFT JOIN PERSONNEL_ALIAS PA_LIC
ON PERSONNEL.PERSON_ID = PA_LIC.PERSON_ID
AND PA_LIC.PERSONNEL_ALIAS_TYPE_CD = 1087
AND PA_LIC.ALIAS_POOL_CD = 683988
AND PA_LIC.ACTIVE_IND = 1
WHERE PERSONNEL.ACTIVE_IND = 1 AND PERSONNEL.PHYSICIAN_IND = 1;
This works very nicely. Where I run into problems is when I put it into Access. I know, I know, Access Sucks. Sometimes one needs to use it, especially if one has multiple database types that they just want to store a few queries in, and especially if one's boss only knows Access. Anyway, I was having trouble with the ANDs inside the FROM, so I moved those to the WHERE, but for some odd reason, Access isn't doing the LEFT JOINs, returning only those personnel with EID, IDTWO, and LICENSENO's. Not everybody has all three of these.
Best shot in Access so far is:
SELECT PERSONNEL.PERSON_ID,
PERSONNEL.NAME_LAST_KEY,
PERSONNEL.NAME_FIRST_KEY,
PA_EID.ALIAS AS EID,
PA_IDTWO.ALIAS AS ID2,
PA_LIC.ALIAS AS LICENSENO
FROM ((PERSONNEL
LEFT JOIN PERSONNEL_ALIAS AS PA_EID ON PERSONNEL.PERSON_ID=PA_EID.PERSON_ID)
LEFT JOIN PERSONNEL_ALIAS AS PA_IDTWO ON PERSONNEL.PERSON_ID=PA_IDTWO.PERSON_ID)
LEFT JOIN PERSONNEL_ALIAS AS PA_LIC ON PERSONNEL.PERSON_ID=PA_LIC.PERSON_ID
WHERE (((PERSONNEL.ACTIVE_IND)=1)
AND ((PERSONNEL.PHYSICIAN_IND)=1)
AND ((PA_EID.PRSNL_ALIAS_TYPE_CD)=1086)
AND ((PA_EID.ALIAS_POOL_CD)=3796547)
AND ((PA_IDTWO.PRSNL_ALIAS_TYPE_CD)=3839085)
AND ((PA_IDTWO.ACTIVE_IND)=1)
AND ((PA_LIC.PRSNL_ALIAS_TYPE_CD)=1087)
AND ((PA_LIC.ALIAS_POOL_CD)=683988)
AND ((PA_LIC.ACTIVE_IND)=1));
I think that part of the problem could be that I'm using the same alias (lookup) table for all three joins. Maybe there's a more efficient way of doing this? Still new to SQL land, so any tips as far as that goes would be great. I feel like these should be equivalent, but the Toad query gives me back many many tens of thousands of imperfect rows, and Access gives me fewer than 500. I need to find everybody so that nobody is left out. It's almost as if the LEFT JOINs aren't working at all in Access.
To understand what you are doing, let's look at simplified version of your query:
SELECT PERSONNEL.PERSON_ID,
PA_EID.ALIAS AS EID
FROM PERSONNEL
LEFT JOIN PERSONNEL_ALIAS AS PA_EID ON PERSONNEL.PERSON_ID=PA_EID.PERSON_ID
WHERE PERSONNEL.ACTIVE_IND=1
AND PERSONNEL.PHYSICIAN_IND=1
AND PA_EID.PRSNL_ALIAS_TYPE_CD=1086
AND PA_EID.ALIAS_POOL_CD=3796547
If the LEFT JOIN finds match, your row might look like this:
Person_ID EID
12345 JDB
If it doesn't find a match, (disregard the WHERE clause for a second), it could look like:
Person_ID EID
12345 NULL
When you add the WHERE clauses above, you are telling it to only find records in the PERSONNEL_ALIAS table that meet the condition, but if no records are found, then the values are considered NULL, so they will never satisfy the WHERE condition and no records will come back...
As Joe Stefanelli said in his comment, adding a WHERE clause to a LEFT JOIN'ed table make it act as an INNER JOIN instead...
Further to #Sparky's answer, to get the equivalent of what you're doing in Oracle, you need to filter rows from the tables on the "outer" side of the joins before you join them. One way to do this might be:
For each table on the "outer" side of a join that you need to filter rows from (that is, the three instances of PERSONNEL_ALIAS), create a query that filters the rows you want. For example, the first query (say, named PA_EID) might look something like this:SELECT PERSONNEL_ALIAS.* FROM PERSONNEL_ALIAS WHERE PERSONNEL_ALIAS.PERSONNEL_ALIAS_TYPE_CD = 1086 AND PERSONNEL_ALIAS.ALIAS_POOL_CD = 3796547
In your "best shot in Access so far" query in the original post: a) replace each instance of PERSONNEL_ALIAS with the corresponding query created in Step 1, and, b) remove the corresponding conditions (on PA_EID, PA_IDTWO, and PA_LIC) from the WHERE clause.