I needed to perform a right outer join in Mongo db. I know we can use $lookup to perform left outer join. I needed to know is there any way to perform right outer join in Mongo db?
Table A Left Outer Join Table Band Table B Right Outer Join Table A are equivalent in the SQL world only difference you will have is the order of the columns.
Similarly Use $lookup in mongodb which performs a left outer join. Since your requirement is to perform Right Outer Join, reverse the order of the collections.
Related
Redshift documentation says:
Merge Join
Typically the fastest join, a merge join is used for inner joins and
outer joins. The merge join is not used for full joins.
But I've always read that full joins and outer joins are the same thing: rows from both tables are kept, regardless of if they exist in the other table.
Are they just referring to left outer joins and right outer joins as those that work for merge sort, while "outer join"s (full outer joins) do not?
Good spot, perhaps the docs could be clearer. You can submit a pull request for our docs if you feel motivated to do so.
In this case, the doc means that a merge join is used when one table is the "primary" table, e.g, it will be used for INNER JOIN, LEFT [OUTER] JOIN, RIGHT [OUTER] JOIN but not used for FULL [OUTER] JOIN where rows from both side must be retained.
I know how the
Nested Join
Merge Join
Hash Join
works and its functionality.
I wanted to know in which situation these joins are used in Postgres
The following are a few rules of thumb:
Nested loop joins are preferred if one of the sides of the join has few rows. Nested loop joins are also used as the only option if the join condition does not use the equality operator.
Hash Joins are preferred if the join condition uses an equality operator and both sides of the join are large and the hash fits into work_mem.
Merge Joins are preferred if the join condition uses an equality operator and both sides of the join are large, but can be sorted on the join condition efficiently (for example, if there is an index on the expressions used in the join column).
A typical OLTP query that chooses only one row from one table and the associated rows from another table will always use a nested loop join as the only efficient method.
Queries that join tables with many rows (which cannot be filtered out before the join) would be very inefficient with a nested loop join and will always use a hash or merge join if the join condition allows it.
The optimizer considers each of these join strategies and uses the one that promises the lowest costs. The most important factor on which this decision is based is the estimated row count from both sides of the join. Consequently, wrong optimizer choices are usually caused by misestimates in the row counts.
Take this sample code...
SELECT Persons.name,
getCarModelID(Persons.ID) AS car_model -- < A function
FROM Persons
LEFT OUTER JOIN Cars ON getCarModelID(Persons.ID) = Cars.ID
In the sample above, is it correct to use "LEFT OUTER JOIN"?
If you are planning to join a table with a function then you will need to use the T-SQL "Outer Apply" operator. Similar to Left Join you used and the above will be possible.
Joins can only join two or more tables but not a table with a function.
You can learn using Apply from this link.
The correct code will be as:
SELECT Persons.name,
getCarModelID(Persons.ID) AS car_model -- < A function
FROM Persons
OUTER APPLY Cars ON getCarModelID(Persons.ID) = Cars.ID;
Why is it that in PostgreSQL you cannot use:
FULL OUTER JOIN
. . .
ON POSITION(table1.column1 IN table2.column1) <> 0,
But you can accomplish the same thing with a left join and a right join and then using a union all to join the results. It's the same exact result set that I want and I feel it should be possible since it's possible to just do the right and left join manually. I can live with having to do it, but it'd be a lot simpler to write with just using a Full Outer Join.
First Query:
Select * from table1 inner join table2 on table1.Id = table2.Id
Second Query:
Select * from table1, table2 where table1.Id = table2.Id
What is difference between these query regarding performance which should one use?
The two statements you posted are logically identical. There isn't really a
practical reason to prefer one over the other, it's largely a matter of
personal style and readability. Some people prefer the INNER JOIN syntax and
some prefer just to use WHERE.
Refering to Using Inner Joins:
In the ISO standard, inner joins can
be specified in either the FROM or
WHERE clause. This is the only type of
join that ISO supports in the WHERE
clause. Inner joins specified in the
WHERE clause are known as old-style
inner joins.
Refering to Join Fundamentals:
Specifying the join conditions in the
FROM clause helps separate them from
any other search conditions that may
be specified in a WHERE clause, and is
the recommended method for specifying
joins.
Personaly, I prefer using INNER JOIN. I find it much clearer, as I can separate the join conditions from the filter conditions and using a seperate join block for each joined table.
To amplify #Akram's answer - many people prefer the inner join syntax, since it then allows you to more easily distinguish between the join conditions (how the various tables in the FROM clause relate to each other) from the filter conditions (those conditions that should be used to reduce the overall result set. There's no difference between them in this circumstance, but on larger queries, with more tables, it may improve readability to use the inner join form.
In addition, once you start considering outer joins, you pretty well need to use the infix join syntax (left outer join,right outer join), so many find a form of symmetry in using the same style for inner join. There is an older deprecated syntax for performing outer joins in the WHERE clause (using *=), but support for such joins is dying out.