I'm trying to JOIN 2 tables ON a key like
SELECT column1,column2
FROM Table1 t1
INNER JOIN Table2 t2 ON t1.t2id = t2.id
Now, I have a 3rd table that has a Foreign Key with t2's id that I want to join... When I do
LEFT JOIN
Table3 t3 ON t3.t2id = --<-------------- This is where I'm lost
I don't know if I should do ON t3.t2id = t1.t2id OR ON t3.t2id = t2.id
What I need is the list of t2ids which are still in the picture after the first join. However, it seems as though if I specify either of the above, it will just pull ids from the original table before the first join?
To clarify one more time: I'm trying to essentially do a INNER JOIN of Table1 and Table2, get the resulting table, then get the t2ids of those results and feed them into a final join such that the final result contains all of Table3's rows as well as the data from the first join
You said: "final result contains all of Table3's rows as well as the data from the first join".
It means that you need
Table3 LEFT JOIN <previous results>
instead of
<previous results> LEFT JOIN Table3
The easiest way to write it is to use Common-Table Expressions:
WITH
CTE_InnerJoin
AS
(
SELECT column1, column2, t1.t2id
FROM
Table1 t1
INNER JOIN Table2 t2 ON t1.t2id = t2.id
)
SELECT
CTE_InnerJoin.column1
,CTE_InnerJoin.column2
,Table3....
FROM
Table3
LEFT JOIN CTE_InnerJoin ON CTE_InnerJoin.t2id = Table3.t2id
;
It doesn't matter what column you include in the CTE: t1.t2id or t2.id, the values in them are the same, because they are inner-joined together.
JOINs already do exactly what you want. A JOIN isn't always between two tables. Frequently, it's between the results of previous joins.
SELECT column1,column2
FROM Table1 t1
INNER JOIN Table2 t2 ON t1.t2id = t2.id
LEFT JOIN
Table3 t3 ON t3.t2id = t2.id
At the point at which you're writing the final ON clause here, what you're joining is precisely the results of the previous INNER JOIN on the left and the table Table3 on the right. All of t1, t2 and t3 are in scope within the ON clause, but note that t1 and t2 are now both used as aliases into the same source of rows - the previous INNER JOIN.
As a further example, consider the "diamond join":
SELECT
*
FROM
t1
left join
t2
on
t1.a = t2.b
left join
t3
on
t1.c = t3.d
inner join
t4
on
t2.e = t4.f OR
t3.g = t4.h
This is a way of joining two tables (t1 and t4) based on two alternative joins. Note that in the final inner join, what is on the "left" is the result of already joining tables t1, t2 and t3.
Join the table having foreign key with
Try this....
SELECT column1,column2
FROM Table1 t1
INNER JOIN Table2 t2 ON t1.t2id = t2.id
LEFT join Table3 t3 ON t2.id=t3.t2id
Or Like this.
SELECT t12.column1 ,
t12.column2 ,
t3.*
FROM (
--- INNER JOIN of Table1 and Table2, get the resulting table,
SELECT t1.column1 ,
t2.column2 ,
t1.t2id --- or t2.id doesn't matter because its inner join
FROM Table1 t1
INNER JOIN Table2 t2 ON t1.t2id = t2.id
) T12
LEFT JOIN Table3 T3 ON t3.t2id = t1.t2id --- then get the t2ids of those results
--- and feed them into a final join
--- if you want to get all rows from Table3, Change LEFT JOIN Table3 T3 ON t3.t2id = T1.t2id
--- into RIGHT JOIN Table3 T3 ON t3.t2id = T1.t2id
try this
select t3.*, column1, column2
from
table1 t1 inner join table2 t2 on t1.t2id = t2.id
right outer join table3 t3 on t3.t2id = t2.id
equvalent to
select t3.*, column1, column2
from
table1 t1 inner join table2 t2 on t1.t2id = t2.id
right outer join table3 t3 on t3.t2id = t1.t2id
if you want all rows from table 3 and those matching rows from table1 inner joined to table2 then you can use this syntax:
select t3.*,
column1, column2
from table3 t3
left join table2 t2
inner join table1 t1
on t1.t2id = t2.id
on t3.t2id = t2.id
Related
The first query works to get a value from t1 that is not in t2.
select t1.*
from table1 t1
where t1.id = '1'
and t1.id2 not in (select t2.id2 from table2 t2 where t2.id='1')
But how do how can I do the same thing using this format instead of an inner select?
select t1.*
from table1 t1, table2 t2
where t1.id = '1'
and t2.id = t1.id
and t1.id2 != t2.id2
You can use a LEFT OUTER JOIN:
select t1.*
from table1 t1
LEFT OUTER JOIN table2 t2
ON t1.id = t2.id
WHERE t2.id IS NULL
LEFT OUTER JOIN says to take all results from the LEFT-most table (table1 here) and only those results from your joined table (table2 here) that match the ON condition. By filtering in your WHERE clause to drop any results where table2.id IS NULL you effectively only leave T1 results that are unmatched to T2.
Also try to stay away from the FROM table1, table2 type of implicit join. It's very old school. INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN are better options for nearly any situation.
I want to use the result of a FULL OUTER JOIN as a table to FULL OUTER JOIN on another table. What is the syntax that I should be using?
For eg: T1, T2, T3 are my tables with columns id, name. I need something like:
T1 FULL OUTER JOIN T2 on T1.id = T2.id ==> Let this be named X
X FULL OUTER JOIN T3 on X.id = t3.id
I want this to be achieved so that in the final ON clause, I want the T3.id to match either T1.id or T2.id. Any alternative way to do this is also OK.
SELECT COALESCE(X.id,t3.id) AS id, *-- specific columns here instead of the *
FROM
(
SELECT COALESCE(t1.id,t2.id) AS id, * -- specific columns here instead of the *
FROM T1 FULL OUTER JOIN T2 on T1.id = T2.id
) AS X
FULL OUTER JOIN T3 on X.id = t3.id
Often, chains of full outer joins don't behave quite as expected. One replacements uses left join. This works best when a table has all the ids you need. But you can also construct that:
from (select id from t1 union
select id from t2 union
select id from t3
) ids left join
t1
on ids.id = t1.id left join
t2
on ids.id = t2.id left join
t3
on ids.id = t3.id
Note that the first subquery can often be replaced by a table. If you have such a table, you can select the matching rows in the where clause:
from ids left join
t1
on ids.id = t1.id left join
t2
on ids.id = t2.id left join
t3
on ids.id = t3.id
where t1.id is not null or t2.id is not null or t3.id is not null
You can do it like you suggested, using IN()
FROM T1
FULL OUTER JOIN T2
ON(T1.id = T2.id)
FULL OUTER JOIN T3
ON(T3.ID IN(T2.id,T1.id))
or I think you can do it with a UNION (depends on what you need) :
SELECT * FROM
(SELECT name,id from T1
UNION
SELECT name,id from T2) x
FULL OUTER JOIN T3
ON(t3.id = x.id)
I need to join two pairs of tables. If there is an ID in Table1 that can also be found in Table3 I need to join the tables. If there is no matching ID from Table1 in Table3, I need to not join the tables.
Ex.
If there is at least one id in Table1 in Table3; do something that is effectively this:
SELECT *
FROM Table1 AS t1
INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
LEFT JOIN Table3 AS t3 ON t1.ID = t3.ID
LEFT JOIN Table4 AS t4 ON t3.ID = t4.ID
If there are no IDs that match between Table1 and Table3; do something that is effectively this:
SELECT *
FROM Table1 AS t1
INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
Just translating your question into SQL, you can do this:
IF EXISTS(SELECT * FROM Table1 T1 INNER JOIN Table3 T3 ON T1.ID=T3.ID)
SELECT *
FROM Table1 AS t1
INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
LEFT JOIN Table3 AS t3 ON t1.ID = t3.ID
LEFT JOIN Table4 AS t4 ON t3.ID = t4.ID
ELSE
SELECT *
FROM Table1 AS t1
INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
I want to use a sql query with multiple joins similar to the example below.
SELECT t1.column1, t1.column2, t1.column3
FROM
table1 t1
LEFT JOIN table2 t2 ON (t1.id1 = t2.id)
LEFT JOIN table3 t3 ON (t1.id1 = t3.id)
JOIN table4 t4 ON t1.id2 = t4.id
WHERE
...
Would this give different results than the following query:
SELECT t1.column1, t1.column2, t1.column3
FROM
table1 t1
LEFT JOIN table2 t2 ON (t1.id1 = t2.id)
LEFT JOIN table3 t3 ON (t2.id = t3.id)
JOIN table4 t4 ON t1.id2 = t4.id
WHERE
...
If they are both 'correct' is the second more efficient than the first?
Thanks
The queries are different, so this isn't a performance issue. The difference are these lines:
LEFT JOIN table3 t3 ON (t1.id1 = t3.id)
and
LEFT JOIN table3 t3 ON (t2.id1 = t3.id)
For the first, t3.id needs to only match t1.id. For the second, it needs to match t2.id1, which in turn must also match t1.id. In other words, the second version requires that the id be in both t1 and t2.
This is because of the LEFT JOIN. The queries would be equivalent if they used INNER JOIN.
The second one is more efficient because it will always return the same or less amount of data.
In the first query you are asking for all the records that are both in table1 and table4 + records from table2 if they exists in table1 + records from the table3 if they exists in table1.
In the second query you are asking for all the records that are both in table1 and table4 + records from table2 if they exists in table1 + records from the table3 if they exists BOTH in in table1 and in table2
I have a query in the following format
select
*
from
Table1 t1
inner join Table2 t2
inner join Table3 t3 on t2.ID = t3.ID
on t3.ID = t1.ID
What I do know:
Not providing the last on condition results in an error.
Additionally changing the first join condition from on t2.ID = t3.ID to on t1.ID = t2.ID results in an error that t1.ID could not be bound.
Obviously the above examples are arbitrary and may not actually produce a practically useful result. However, an explanation of what providing the on later is actually doing would be great.
Thanks
EDIT
I'm not trying to change the question to something that works but to understand what MSSQL is doing when I provide it.
You can use the format you specified (presuming the correct table aliases), if you use parenthesis.
Select ... -- never use Select *
From (Table1 As T1
Join Table2 As T2
On T2.ID = T1.ID)
Join Table3 As T3
On T3.ID = T1.ID
However, with equi-joins (inner joins) it really makes no difference and it is easier to read if you do not use parenthesis. However, this format is very useful with outer joins. Take the following two examples:
Example 1
Select ...
From Table1 As T1
Left Join Table2 As T2
On T2.T1_ID = T1.ID
Join Table3 As T3
On T3.T2_ID = T2.ID
Example 2
Select ...
From Table1 As T1
Left Join (Table2 As T2
Join Table3 As T3
On T3.T2_ID = T2.ID)
On T2.T1_ID = T1.ID
Suppose in this situation, that T3.T2_ID is a non-nullable foreign key to Table2. In Example1, the Inner Join to Table3 will effectively filter out rows that would have been null because the given T2.T2_ID does not exist in Table1. However, in the second example, the join between Table2 and Table3 is done before the Left Join to Table1 is processed. Thus, we'll get the same rows from Table1 and Table2 as:
Example 3
Select ...
From Table1 As T1
Left Join Table2 As T2
On T2.T1_ID = T1.ID
Assuming you meant t1 rather than t, then your query:
select
*
from
Table1 t1
inner join Table2 t2
inner join Table3 t3 on t2.ID = t3.ID
on t3.ID = t1.ID
...can be made rather more clear by the addition of the brackets it doesn't really need:
select
*
from
Table1 t1
inner join
(Table2 t2 inner join Table3 t3 on t2.ID = t3.ID) on t3.ID = t1.ID
Effectively, you're explicitly saying "join t2 to t3, then join t1 to that."
Does that help?
First off--you don't define what t is
Table1 is aliased t1
Table2 is aliased t2
Table3 is aliased t3
But there is no plain t.
Second, you are not doing a join of t1 to t2, but of t1 to t3 and then t3 to t2. That will work. If there is a relation between t1 and t2 (t1.ID=t2.ID) then that "on" statement should directly follow the inner join statement for t2:
select
*
from
Table1 t1
inner join Table3 t3 on t1.ID = t3.ID
inner join Table2 t2 on t3.ID = t2.ID
UPDATE (based on your update)
are t1.ID, t2.ID, and t3.ID all the same data type?