Can we get the results of a left outer join using a right outer join?
Yes, you can do this. A right (outer) join is equivalent to a left (outer) join with the position of the tables switched.
Hence, the following query:
SELECT *
FROM table1 t1
LEFT JOIN table2 t2
ON t1.col = t2.col
is equivalent to
SELECT *
FROM table2 t2
RIGHT JOIN table1 t1
ON t1.col = t2.col
Related
I'm using SQL server manger.
I have 3 tables
I need a query that pulls t1 ands add an Origin Basin and a Destination Basin.
So far I have the following:
select T1.[Country (destination)], T3.AreaName
From T1
left outer join T2 on
T1.[Country (destination)] = T2.CountryName
inner join T3 on
T2.AreaID = T3.AreaID
inner join T3 on
T2.AreaID = T3.AreaID
Which returns:
Country | Area
However, I'm having trouble doing this for the second country column. I believe you use aliases. I've tried:
select (select AreaName
FROM T3
where T3.AreaID = T2.AreaID) as 'Area Imp',
(select AreaID
From T2
where T2.CountryName = T1.[Country (origin)]) as 'x',
(select AreaID
From T2
where T2.CountryName = T1.[Country (destination)]) as 'y'
FROM T1
But I can't get it to work.
This is what you need to do:
select t1.date, t1.country_destination, t1.country_origin, destination_area.AreaName as area_destination, origin_area.AreaName as area_origin
from t1 as t1 join t2 as destination on t1.country_destination = destination.countryname
join t2 as origin on t1.country_origin = origin.countryname
join t3 as destination_area on t2.areaid = destination_area.areaid
join t3 as origin_area on t2.areaid = origin_area.areaid
You will need to join with the same table twice, both for t2 and t3 so that you get the matching records for your needs.
It helps usually to put aliases that match the purpose of the join (in this case, destination and origin) when writing the query.
I think what you're trying to do is something like this:
select T1.*, T3dest.AreaName, T3orig.AreaName
From
T1
inner join
T2 T2dest on
T1.[Country (destination)] = T2dest.CountryName
inner join
T3 T3dest on
T2dest.AreaID = T3dest.AreaID
inner join
T2 T2orig on
T1.[Country (origin)] = T2orig.CountryName
inner join
T3 T3orig on
T2orig.AreaID = T3orig.AreaID
Note that I've switched to inner joins throughout, at the moment. If you do want left join semantics, you either need to use those for all of the joins to the T2 and T3 tables or you need to change the join order (so that the relevant T3 joins to the T2 tables occur before the attempted join with T1). It's not clear from the sample data if that's required, however.
Try this, You would still want to join on area id's
select T1.Date,T1.[Country (destination)], null [Country (origin)], T3.AreaName [AreaName(Destination)], null [AreaName(Origin)]
From T1
left outer join T2 on
T1.[Country (destination)] = T2.CountryName
inner join T3 on
T2.AreaID = T3.AreaID
union all
select T1.Date,null [Country (destination)], t1.[Country (origin)], Null [AreaName(Destination)], t3. [AreaName(Origin)]
From T1
left outer join T2 on
T1.[Country (Origin)] = T2.CountryName
inner join T3 on
T2.AreaID = T3.AreaID
Query1: t1 left join t2 on some condition cross join t3 right join t4 on some condition
Query2: t1 left join t2 on some condition cross join t4 left join t3 on some condition
Will the result of these queries be the same or not?
They are not the same.
Ex. if we consider all the tables have the same number of rows, the first query returns less rows than the second one.
This is somewhat of a followon to SQL JOIN where to place the WHERE condition?
I would prefer to use the USING clause on the join, but am not yet able to put the field value condition with the join.
SELECT 1
FROM table1 t1
JOIN table2 t2 USING(id) AND t2.field = 0;
ORA-00933: SQL command not properly ended
Is it possible to have USING and another condition as part of the JOIN clause?
You can use:
SELECT 1
FROM table1 t1
JOIN table2 t2 USING(id)
WHERE t2.field = 0;
Using USING(id) is like using ON t1.id = t2.id except that in the JOIN result instead of two columns t1.id & t2.id there is only one id column.
For INNER JOIN USING with a condition followed by an OUTER JOIN you need a subquery to keep the WHERE with the USING:
SELECT ...
FROM (SELECT id, ...
FROM table1 t1
JOIN table2 t2 USING(id)
WHERE t2.field = 0) s
LEFT JOIN ...;
For an OUTER JOIN USING with a condition you need a subselect:
SELECT ...
FROM table1 t1
LEFT JOIN (SELECT *
FROM table2 t2
WHERE t2.field = 0) t2
USING (id);
See this re ON/WHERE with JOIN. See this re ON/WHERE when mixing INNER & OUTER JOINs.
I've got 3 tables that I want to join and filter on some conditions.
I've first wrote this query:
select * from table1 t1
left join (select * from table2 where table2.fieldX=...) t2
on t1.id_12=t2.id_12
left join table3
on t2.id_23=t3.id_23
where t1.fieldY=...
Then I wanted to make it looks like more canonical by rewritting it like that:
select * from table1 t1
left join table2 t2
on t1.id_12=t2.id_12
left join table3
on t2.id_23=t3.id_23
where table2.fieldX=...
and t1.fieldY=...
But it does not give the same result.
I dont't understand why...
Do you?
Thanks in advance.
When you put table2.fieldX=... in the where clause you eliminate all rows from table1 that do not have a corresponding row in table2. Effectively you are changing the left join into an inner join.
Instead, you can apply the table2 filter in the join itself:
SELECT
*
FROM
Table1 t1
LEFT JOIN Table2 t2 ON t1.id_12 = t2.id_12 AND t2.fieldX = ...
LEFT JOIN Table3 t3 ON t2.id_23 = t3.id_23
WHERE
t1.fieldY = ...
Did you add the inner select where on the second query?
SELECT *
FROM table1 t1
LEFT JOIN table2 t2
on t1.id_12=t2.id_12
LEFT JOIN table3
on t2.id_23=t3.id_23
WHERE t1.fieldY=...
and t2.fieldX=...
I should probably know this by now, but what, if any is the difference between the two statements below?
The nested join:
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
ON t2.table2_ID = t1.table1_ID
The more traditional join:
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
Well, it's the order of operations..
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
ON t2.table2_ID = t1.table1_ID
could be rewritten as:
SELECT
t1.*
FROM
table1 t1 -- inner join t1
INNER JOIN
(table2 t2 LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID) -- with this
ON t2.table2_ID = t1.table1_ID -- on this condition
So basically, first you LEFT JOIN t2 with t3, based on the join condition: table3_ID = table2_ID, then you INNER JOIN t1 with t2 on table2_ID = table1_ID.
In your second example you first INNER JOIN t1 with t2, and then LEFT JOIN the resulting inner join with table t3 on the condition table2_ID = table1_ID.
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
could be rewritten as:
SELECT
t1.*
FROM
(table1 t1 INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID) -- first inner join
LEFT JOIN -- then left join
table3 t3 ON t3.table3_ID = t2.table2_ID -- the result with this
EDIT
I apologize. My first remark was wrong. The two queries will produce the same results but there may be a difference in performance as the first query may perform slower than the second query in some instances ( when table 1 contains only a subset of the elements in table 2) as the LEFT JOIN will be executed first - and only then intersected with table1. As opposed to the second query which allows the query optimizer to do it's job.
For your specific example, I don't think there should be any difference in the query plans generated, but there's definitely a difference in readability. Your 2nd example is MUCH easier to follow.
If you were to reverse the types of joins in the example, you could end up with much different results.
SELECT t1.*
FROM table1 t1
LEFT JOIN table2 t2 ON t2.table2_ID = t1.table1_ID
INNER JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
-- may not produce the same results as...
SELECT t1.*
FROM table1 t1
LEFT JOIN table2 t2
INNER JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
ON t2.table2_ID = t1.table1_ID
Based on the fact that order of the joins DOES matter in many cases - careful thought should go into how you're writing your join syntax. If you find that the 2nd example is what you're really trying to accomplish, i'd consider rewriting the query so that you can put more emphasis on the order of your joins...
SELECT t1.*
FROM table2 t2
INNER JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
RIGHT JOIN table1 t1 ON t2.table2_ID = t1.table1_ID
The best way to see what is different in these two queries is to compare the Query Plan for both these queries.
There is no difference in the result sets for these IF there are always rows in table3 for a given row in table2.
I tried it on my database and the difference in the query plans was that
1. For the first query, the optimizer chose to do the join on table2 and table 3 first.
2. For the second query, the optimizer chose to join table1 and table2 first.
You should see no difference at all between the two queries, provided your DBMS' optimizer is up to scratch. That, however, even for big-iron, high-cost platforms, is not an assumption I'd be confident in making, so I'd be fairly unsurprised to discover that query plans (and consequently execution times) varied.