here is my query:
select p1.driver_id, p1.name, o1.driver_id, o1.license, part.license, part.amount
from
(person p1 full outer join owns o1 on p1.driver_id = o1.driver_id) and
(Participated part full outer join ol on part.license = o1.license)
I get the sql statement not properly ended error. I just want to build the table as part of my process of solving a more complicated problem.
That is not how you define the relations in the FROM part. Try the following:
select p1.driver_id, p1.name, o1.driver_id, o1.license, part.license, part.amount
from
person p1
full outer join owns o1 on p1.driver_id = o1.driver_id
full outer join Participated part on part.license = o1.license;
Notes:
After FROM you can list some relations (comma separated), and a relation can also be a a FULL OUTER JOIN b ON ... form as you correctly wrote.
You can also chain these (with brackets if you prefer) as
a FULL OUTER JOIN b ON ... FULL OUTER JOIN c ON ...
You can not use AND to connect relations, that is only used to connect conditions.
You can bracket a statement to form a subquery, such as
(SELECT * FROM a FULL OUTER JOIN b ON ...) AS t
but note t hat you need the SELECT * at the beginning to turn it into a relation and in most systems you need an alias after (t in the example).
SELECT p1.driver_id, p1.name, o1.driver_id, o1.license, part.license, part.amount
FROM person p1
FULL OUTER JOIN owns o1
ON p1.driver_id = o1.driver_id
FULL OUTER JOIN Participated part
ON part.license = o1.license
Related
I have a complex query with multiple left joins and subqueries which I need to implement in Entify Framework. I've received the monster SQL and
my goal is to do it on a elegant way with EF. The query consumes multiple tables and creates a "WITH" subquery on top which
is included in the joins later. I've done a first approach with EF but when I inspect the output that EF sends to the DB, inner joins are sent when
I am expecting LEFT JOINs.
A summary of the SQL follows:
WITH SUB_QUERY
AS ( SELECT FIELD_A,
FIELD_B,
FIELD_C,
MAX (FIELD_D) MAX_FIELD_D
FROM TABLE_X
WHERE SOME FIELD_A = 'WHATEVER'
GROUP BY FIELD_A, FIELD_B, FIELD_C)
SELECT C.FIELD_A,
C.FIELD_B,
B.FIELD_X,
D.FIELD_S,
E.FIELD_J,
F.FIELD_Y
FROM TABLE_A A
LEFT JOIN SUB_QUERY B
ON A.FIELD_C = B.FIELD_C
LEFT JOIN TABLE_C C
ON B.FIELD_A = C.FIELD_A
LEFT JOIN TABLE_D D
ON A.FIELD_C = D.FIELD_C
LEFT JOIN TABLE_E E
ON A.FIELD_X = E.FIELD_X
LEFT JOIN TABLE_F F
ON A.FIELD_W = F.FIELD_W
WHERE A.FIELD_H = D.FIELD_H
AND A.FIELD_D = B.MAX_FIELD_D
As you see, a subquery on top filters and groups some data to be consumed in a join below. Then all the joins take place
and some fields are taken from different tables as the output of the query.
Which approach would you recommend me to accomplish this task? I've tried different approaches and no one of them works (either retrieve nothing, or many more rows than the SQL query on the DB, etc..)
Please note that the Domain Model in Entity Framework is properly setup: Primary Keys, collections, nested objects etc.. so I believe some of these
joins are not even required because my EF entities contain already references to the child collections and parent objects (navigation properties).
Thanks a lot!!
if you really need a left join you should mode the where condition related to a left joined table in the proper on clause
FROM TABLE_A A
LEFT JOIN SUB_QUERY B
ON A.FIELD_C = B.FIELD_C
LEFT JOIN TABLE_C C
ON B.FIELD_A = C.FIELD_A
LEFT JOIN TABLE_D D
ON A.FIELD_C = D.FIELD_C AND A.FIELD_D = B.MAX_FIELD_D
LEFT JOIN TABLE_E E
ON A.FIELD_X = E.FIELD_X
LEFT JOIN TABLE_F F
ON A.FIELD_W = F.FIELD_W
the use of a left join table column in where force the relation to work as a INNER JOIN
I remember this rule of thumb from back in college that if you put a left join in a SQL query, then all subsequent joins in that query must also be left joins instead of inner joins, or else you'll get unexpected results. But I don't remember what those results are, so I'm wondering if maybe I'm misremembering something. Anyone able to back me up on this or refute it? Thanks! :)
For instance:
select * from customer
left join ledger on customer.id= ledger.customerid
inner join order on ledger.orderid = order.id -- this inner join might be bad mojo
Not that they have to be. They should be (or perhaps a full join at the end). It is a safer way to write queries and express logic.
Your query is:
select *
from customer c left join
ledger l
on c.id = l.customerid inner join
order o
on l.orderid = o.id
The left join says "keep all customers, even if there is no matching record in ledger. The second says, "I have to have a matching ledger record". So, the inner join converts the first to an inner join.
Because you presumably want all customers, regardless of whether there is a match in the other two tables, you would use a left join:
select *
from customer c left join
ledger l
on c.id = l.customerid left join
order o
on l.orderid = o.id
You remember correctly some parts of it!
The thing is, when you chain join tables like this
select * from customer
left join ledger on customer.id= ledger.customerid
inner join order on ledger.orderid = order.id
The JOIN is executed sequentialy, so when customer left join ledger happens, you are making sure all joined keys from customer return (because it's a left join! and you placed customers to the left).
Next,
The results of the former JOIN are joined with order (using inner join), forcing the "the first join keys" to match (1 to 1) with the keys from order so you will end up only with records that were matched in order table as well
Bad mojo? it really depends on what you are trying to accomplish.
If you want to guarantee all records from customers return, you should keep "left joining" to it.
You can, however, make this a little more intuitive to understand (not necessarily a better way of writing SQL!) by writing:
SELECT * FROM
(
(SELECT * from customer) c
LEFT JOIN
(SELECT * from ledger) l
ON
c.id= l.customerid
) c_and_l
INNER JOIN (OR PERHAPS LEFT JOIN)
(SELECT * FROM order) as o
ON c_and_l.orderid (better use c_and_l.id as you want to refer to customerid from customers table) = o.id
So now you understand that c_and_l is created first, and then joined to order (you can imagine it as 2 tables are joining again)
I am still messing around with this query, which is working but is not returning the data I need.
SELECT prod.Code,
prod.Description,
prod.Groupp,
prod.Quantity,
prod.Unit,
prod.Standard,
prod.Type,
prod.Model,
prod.GroupBKM,
prod.Note,
comp.Unit,
comp.Cost
FROM dbo.Product1 prod
INNER JOIN dbo.Components comp
ON comp.Code = prod.Code
The above query is returning the data only if a comp.code=prod.code exists while I would like to get the data prod.* in any case and obviously the data relevant comp.cost, if does not exist, will be null.
I cannot get it right! Any help will be appreciated.
Replace INNER JOIN with LEFT JOIN
SELECT prod.Code,
prod.Description,
prod.Groupp,
prod.Quantity,
prod.Unit,
prod.Standard,
prod.Type,
prod.Model,
prod.GroupBKM,
prod.Note,
comp.Unit,
comp.Cost
FROM dbo.Product1 prod
LEFT JOIN dbo.Components comp
ON comp.Code = prod.Code
By definition you cannot do this with an INNER JOIN because an INNER JOIN is defined as only returning items for which a match was found.
If you want to return rows in the base SELECT even if the JOIN predicate fails, then you want a LEFT OUTER JOIN ... which is defined as precisely that.
From Wikipedia:
An outer join does not require each record in the two joined tables to
have a matching record. The joined table retains each record—even if
no other matching record exists. Outer joins subdivide further into
left outer joins, right outer joins, and full outer joins, depending
on which table's rows are retained (left, right, or both).
In your case, replace INNER JOIN with LEFT OUTER JOIN.
I have the following tables with the following attributes:
Op(OpNo, OpName, Date)
OpConvert(OpNo, M_OpNo, Source_ID, Date)
Source(Source_ID, Source_Name, Date)
Fleet(OpNo, S_No, Date)
I have the current multiple JOIN query which gives me the results that I want:
SELECT O.OpNo AS Op_NO, O.OpName, O.Date AS Date_Entered, C.*
FROM Op O
LEFT OUTER JOIN OpConvert C
ON O.OpNo = C.OpNo
LEFT OUTER JOIN Source D
ON C.Source_ID = D.Source_ID
WHERE C.OpNo IS NOT NULL
The problem is this. I need to join the Fleet table on the previous multiple JOIN statement to attach the relevant S_No to the multiple JOIN table. Would I still be able to accomplish this using a LEFT OUTER JOIN or would I have to use a different JOIN statement? Also, which table would I JOIN on?
Please note that I am only familiar with LEFT OUTER JOINS.
Thanks.
I guess in your case you could use INNER JOIN or LEFT JOIN (which is the same thing as LEFT OUTER JOIN in SQL Server.
INNER JOIN means that it will only return records from other tables only if there are corresponding records (based on the join condition) in the Fleet table.
LEFT JOIN means that it will return records from other tables even if there are no corresponding records (based on the join condition) in the Fleet table. All columns from Fleet will return NULL in this case.
As for which table to join, you should really join the table that makes more logical sense based on your data structure.
Yes, you can use all tables mentioned before in your join conditions. Actually, JOINS (no matter of INNER, LEFT OUTER, RIGHT OUTER, CROSS, FULL OUTER or whatever) are left- associative, i. e. they are implicitly evaluated as if they would have been included in parentheses from the left as follows:
FROM ( ( ( Op O
LEFT OUTER JOIN OpConvert C
ON O.OpNo = C.OpNo
)
LEFT OUTER JOIN Source D
ON C.Source_ID = D.Source_ID
)
LEFT OUTER JOIN Fleet
ON ...
)
This is similar to how + or - would implicitly use parentheses, i. e.
2 + 3 - 4 - 5
is evaluated as
(((2 + 3) - 4) - 5)
By the way: If you use C.OpNo IS NOT NULL, then the LEFT OUTER JOIN Source D is treated as if it were an INNER JOIN, as you are explicitly removing all the "OUTER" rows.
I have three tables: R, S and P.
Table R Joins with S through a foreign key; there should be at least one record in S, so I can JOIN:
SELECT
*
FROM
R
JOIN S ON (S.id = R.fks)
If there's no record in S then I get no rows, that's fine.
Then table S joins with P, where records is P may or may not be present and joined with S.
So I do
SELECT
*
FROM
R
JOIN S ON (S.id = R.fks)
LEFT JOIN P ON (P.id = S.fkp)
What if I wanted the second JOIN to be tied to S not to R, like if I could use parentheses:
SELECT
*
FROM
R
JOIN (S ON (S.id = R.fks) JOIN P ON (P.id = S.fkp))
Or is that already a natural behaviour of the cartesian product between R, S and P?
All kinds of outer and normal joins are in the same precedence class and operators take effect left-to-right at a given nesting level of the query. You can put the join expression on the right side in parentheses to cause it to take effect first. Remember that you will have to move the ON clauses around so that they stay with their joins—the join in parentheses takes its ON clause with it into the parentheses, so it now comes textually before the other ON clause which will be after the parentheses in the outer join statement.
(PostgreSQL example)
In
SELECT * FROM a LEFT JOIN b ON (a.id = b.id) JOIN c ON (b.ref = c.id);
the a-b join takes effect first, but we can force the b-c join to take effect first by putting it in parentheses, which looks like:
SELECT * FROM a LEFT JOIN (b JOIN c ON (b.ref = c.id)) ON (a.id = b.id);
Often you can express the same thing without extra parentheses by moving the joins around and changing the direction of the outer joins, e.g.
SELECT * FROM b JOIN c ON (b.ref = c.id) RIGHT JOIN a ON (a.id = b.id);
When you join the third table, your first query
SELECT
*
FROM
R
JOIN S ON (S.id = R.fks)
is like a derived table to which you're joining the third table. So if R JOIN S produces no rows, then joining P will never yield any rows (because you're trying to join to an empty table).
So, if you're looking for precedence rules then in this case it's just set by using LEFT JOIN as opposed to JOIN.
However, I may be misunderstanding your question, because if I were writing the query, I would swap S and R around. eg.
SELECT
*
FROM
S
JOIN R ON (S.id = R.fks)
The second join is tied to S as you explicity state JOIN P ON (P.id = S.fkp) - no column from R is referenced in the join.
with a as (select 1 as test union select 2)
select * from a left join
a as b on a.test=b.test and b.test=1 inner join
a as c on b.test=c.test
go
with a as (select 1 as test union select 2)
select * from a inner join
a as b on a.test=b.test right join
a as c on b.test=c.test and b.test=1
Ideally, we would hope that the above two queries are the same. However, they are not - so anybody that says a right join can be replaced with a left join in all cases is wrong. Only by using the right join can we get the required result.