There are two massive tables from where I have to query out a subset of interest. Both have multiple common columns but with lot of nulls. I want to join with multiple join conditions on these columns and then combine the result sets. Using Union method is costing too much and db is not ready to allow the query. Could someone help how I can optimize with some smart technique.
My query is like
select col1,col2,col3,col4,col5 from tab1 T1
left join tab2 T2 on T1.col1=T2.col1
Union
select col1,col2,col3,col4,col5 from tab1 T1
left join tab2 T2 on T1.col2=T2.col2
Union
select col1,col2,col3,col4,col5 from tab1 T1
left join tab2 T2 on T1.col3=T2.col3
Thanks for your support.
I have a table that has multiple values saved in an array in one column. (I know that this is not normalized/optimal database structure.) I'm trying to write a query that can create rows for each value in the array. The query below is working for me in Tableau but not Google Data Studio (I'm using a custom query with the PostgreSQL connector). Are there any limitations/different syntax requirements when using UNNEST in Data Studio?
SELECT
e.name as event_name,
e.date as event_date,
l.full_name as leader_name,
p.full_name as participant_name
FROM
(
SELECT
event_id,
user_id,
UNNEST(participants_ids)::INTEGER as participant_id
from event_reports
) r
LEFT JOIN events e ON r.event_id = e.id
LEFT JOIN users l ON r.user_id = l.id
LEFT JOIN users p ON r.participant_id = p.id
Your inner query should probably have a lateral join:
SELECT e.event_id,
e.user_id,
p.participant_id
FROM event_reports AS e
CROSS JOIN
LATERAL unnest(e.participants_ids) AS p(as participant_id)
With a lateral join, you can reference something from the left side of the join on the right side. The cross join combines each event_reports row with rach participant ID that belongs to that row.
I have two tables table1(id,name,type) and table2(id,source,destination)
When I run query
SELECT
name,
source,
destination
FROM
table1,
table2
WHERE
table1.id=table2.id
If there's no id matching between two tables, can I still get empty column for source and destination .
Yes, you basically want an OUTER JOIN and remember to always use the explicit ANSI JOIN syntax and not the implicit comma syntax for joins.Also use proper table aliases to avoid ambiguity.
SELECT
t1.name,
t2.source,
t2.destination
FROM
table1 t1 left outer join
table2 t2 ON t1.id = t2.id
Apology in advance for a long question, but doing this just for the sake of learning:
i'm new to SQL and researching on JOIN for now. I'm getting two different behaviors when using INNER and OUTER JOIN. What I know is, INNER JOIN gives an intersection kind of result while returning only common rows among tables, and (LEFT/RIGHT) OUTER JOIN is outputting what is common and remaining rows in LEFT or RIGHT tables, depending upon LEFT/RIGHT clause respectively.
While working with MS Training Kit and trying to solve this practice: "Practice 2: In this practice, you identify rows that appear in one table but have no matches in another. You are given a task to return the IDs of employees from the HR.Employees table who did not handle orders (in the Sales.Orders table) on February 12, 2008. Write three different solutions using the following: joins, subqueries, and set
operators. To verify the validity of your solution, you are supposed to return employeeIDs: 1, 2, 3, 5, 7, and 9."
I'm successful doing this with subqueries and set operators but with JOIN is returning something not expected. I've written the following query:
USE TSQL2012;
SELECT
E.empid
FROM
HR.Employees AS H
JOIN Sales.Orders AS O
ON H.empid = O.empid
AND O.orderdate = '20080212'
JOIN HR.Employees AS E
ON E.empid <> H.empid
ORDER BY
E.empid
;
I'm expecting results as: 1, 2, 3, 5, 7, and 9 (6 rows)
But what i'm getting is: 1,1,1,2,2,2,3,3,3,4,4,5,5,5,6,6,7,7,7,8,8,9,9,9 (24 rows)
I tried some videos but could not understand this side of INNER/OUTER JOIN. I'll be grateful if someone could help this side of JOIN, why is it so and what should I try to understand while working with JOIN.
you can also use left outer join to get not matching
*** The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match.
SELECT
H.empid
FROM
HR.Employees AS H
LEFT OUTER JOIN Sales.Orders AS O
ON H.empid = O.empid
AND O.orderdate = '20080212'
WHERE O.empid IS NULL
Above script will return emp id who did not handle orders on specify date
here you can see all kind of join
Diagram taken from: http://dsin.wordpress.com/2013/03/16/sql-join-cheat-sheet/
adjust your query to be like this
USE TSQL2012;
SELECT
E.empid
FROM
HR.Employees AS H
JOIN Sales.Orders AS O
ON H.empid = O.empid
where O.orderdate = '2008-02-12' AND O.empid IN null
ORDER BY
E.empid
;
USE TSQL2012;
SELECT
distinct E.empid
FROM
HR.Employees AS H
JOIN Sales.Orders AS O
ON H.empid = O.empid
AND O.orderdate = '20080212'
JOIN HR.Employees AS E
ON E.empid <> H.empid
ORDER BY
E.empid
;
Primary things to always remind yourself when working with SQL JOINs:
INNER JOINs require a match in the join in order for result set rows produced prior to the INNER JOIN to remain in the result set. When no match is found for a row, the row is discarded from the result set.
For a row fed to an INNER JOIN that matches to ONLY one row, only one copy of that row fed to the result set is delivered.
For a row fed to an INNER JOIN that matches to multiple rows, the row will be delivered multiple times, once for each row match from the INNER JOIN table.
OUTER JOINs will not discard rows fed to them in the result set, whether or not the OUTER JOIN results in a match or not.
Just like INNER JOINs, if an OUTER JOIN matches to more than one row, it will increase the number of rows in the result set by duplicating rows equal to the number of rows matched from the OUTER JOIN table.
Ask yourself "if I get NO match on the JOIN, do I want the row discarded or not?" If the answer is NO, use an OUTER JOIN. If the answer is YES, use an INNER JOIN.
If you don't need to reference any of the columns from a JOIN table, don't perform a JOIN at all. Instead, use a WHERE EXISTS, WHERE NOT EXISTS, WHERE IN, WHERE NOT IN, etc. or similar, depending on your database engine in use. Don't rely on the database engine to be smart enough to discard unreferenced columns resulting from JOINs from the result set. Some databases may be smart enough to do that, some not. There's no reason to pull columns into a result set only to not reference them. Doing so increases chance of reduced performance.
Your JOIN of:
JOIN HR.Employees AS E
ON E.empid <> H.empid
...is matching to all Employees rows with a DIFFERENT EMPID to all rows fed to that join. Use of NOT EQUAL on an INNER JOIN is a very rare thing to do or need, especially if the JOIN predicate is testing only ONE condition. That is why your getting duplicate rows in the result set.
On DB2, we could perform an EXCEPTION JOIN to accomplish that using a JOIN alone. Normally, on DB2, I would use a WHERE NOT EXISTS for that. On SQL Server you could do a JOIN to a query where the query set is all employees without orders in SALES.ORDERS on the specified date, but I don't know if that violates the rules of your tutorial.
Naveen posted the solution it appears your tutorial is looking for!
I have a Q about INTERSECT clause between two select statements in Sql server 2008.
Select 1 a,b,c ..... INTERSECT Select 2 a,b,c....
Here, the datasets of the two queries should exactly match to return the common elements.
But, I want only column a of both select statements to match.
If the values of column a in both the queries have same values, the entire row should appear in the result set.
Can i Do that and How ??
Thanks,
Marcus..
The best thing to do is to look at the queries itself. DO they need an INTERSECT, of is it possible to make a join with it
for example.
An INTERSECT looks like this
select columnA
from tableA
INTERSECT
select columnAreference
from tableB
Your result would have all columns that are in BOTH tables.. so a join would be more usefull
select columnA
from tableA a
inner join tableB b
on b.columnAReference = a.columnA
If you look into the execution plan you'll see that the INTERSECT will do a left semi join and the inner join will do a, like expected, an inner join. A left semi join isn't something you can tell the query optimizer to do, BUT IT IS FASTER!!!! A left semi join will only return 1 row from the left table, where a normal join will return them all. In this particular case it will be faster.
So an INTERSECT isn't a bad thing which should be eliminated with an INNER JOIN construction, sometimes it will perform even better.
However, to give you the best answer, i will need some more details about your query :)
select * from table1 t1 inner join Table2 t2
on t1.col1=t2.col1