Left Outer join: Or simply called as Left Join. It returns all the rows present in the Left table and matching rows from the right table if any. It returns matching rows from the left table if any , and all the rows present in the Right table. Matched ids and not matched rows from both Tables are shown. Mostly written as just JOIN in sql queries. It returns only the matching records between the tables. Can also check DEMO here. There are a lot of good answers here with very accurate relational algebra examples.
Here is a very simplified answer that might be helpful for amateur or novice coders with SQL coding dilemmas. Hop into psql and create a tiny database of cats and humans.
You can just copy-paste this whole section. Any human without a cat or cat without a human is excluded. Any cat without a human is excluded. Any human without a cat is excluded. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?
Collectives on Stack Overflow. Learn more. Ask Question. Asked 13 years, 2 months ago. Active 2 months ago. Viewed 2. Improve this question. Martin Smith k 80 80 gold badges silver badges bronze badges. Chris de Vries Chris de Vries The area unique to each circle represents the set of rows you get by taking its table's rows that don't participate in A JOIN B and adding the columns unique to the other table all set to NULL. And most give a vague bogus correspondence of the circles to A and B.
DanteTheSmith No, that suffers from the same problems as the diagrams here. Venn diagrams show elements in sets. Just try to identify exactly what the sets are and what the elements are in these diagrams. The sets aren't the tables and the elements aren't their rows. All bogus. You are doing just what thousands of others have done--got a vague impression you wrongly assume makes sense. My preceding comment is about a confused repudiated Jeff Atwood blog post.
My 1st comment's link is external, but i. Add a comment. Active Oldest Votes. Assuming you're joining on columns with no duplicates, which is a very common case: An inner join of A and B gives the result of A intersect B, i. Examples Suppose you have two tables, with a single column each, and data as follows: A B - - 1 3 2 4 3 5 4 6 Note that 1,2 are unique to A, 3,4 are common, and 5,6 are unique to B. Inner join An inner join using either of the equivalent queries gives the intersection of the two tables, i.
Improve this answer. Darryl Hein k 88 88 gold badges silver badges bronze badges. Mark Harrison Mark Harrison k gold badges silver badges bronze badges.
It would be good to augment the example by adding another row in table B with value 4. This will show that inner joins need not be on equal no of rows. An excellent explanation, however this statement: An outer join of A and B gives the results of A union B, i. An outer join will give the results of A intersect B in addition to one of the following: all of A left join , all of B right join or all of A and all of B full join.
Only this last scenario is really A union B. Still, a well written explanation. Ameer, Thanks. I have downvoted this because it is wrong. Please consider removing the answer as it will mislead generations of computer science students who are fooled by the large upcote count. Venn diagrams do not explain join. The inner part of a join is not intersection. Show 8 more comments. The Venn diagrams don't really do it for me. Imagine a cross join.
Evaluate the on clause against all rows from step 1 keeping those where the predicate evaluates to true For outer joins only add back in any outer rows that were lost in step 2. NB: In practice the query optimiser may find more efficient ways of executing the query than the purely logical description above but the final result must be the same I'll start off with an animated version of a full outer join.
Colour, B. Inner Join. Evaluate the condition in the "ON" clause for all rows in the cross join result. If true return the joined row. Otherwise discard it. Left Outer Join. Same as inner join then for any rows in the left table that did not match anything output these with NULL values for the right table columns. Right Outer Join. Same as inner join then for any rows in the right table that did not match anything output these with NULL values for the left table columns. Full Outer Join.
Same as inner join then preserve left non matched rows as in left outer join and right non matching rows as per right outer join. Colour The above is the classic equi join. Colour NOT IN 'Green','Blue' The inner join condition need not necessarily be an equality condition and it need not reference columns from both or even either of the tables.
Colour Outer Joins are logically evaluated in the same way as inner joins except that if a row from the left table for a left join does not join with any rows from the right hand table at all it is preserved in the result with NULL values for the right hand columns. Colour Right outer joins act similarly to left outer joins except they preserve non matching rows from the right table and null extend the left hand columns. Colour Full outer joins combine the behaviour of left and right joins and preserve the non matching rows from both the left and the right tables.
Colour AND B. Community Bot 1 1 1 silver badge. Martin Smith Martin Smith k 80 80 gold badges silver badges bronze badges. I will say that while this doesn't work for me nearly as well as the Venn diagrams, I appreciate that people vary and learn differently and this is a very well presented explanation unlike any I've seen before, so I support ypercube in awarding the bonus points.
Kudos to you, Martin Smith. OldPro The Venn diagrams are OK as far as they go I suppose but they are silent on how to represent a cross join, or to differentiate one kind of join predicate such as equi join from another. The mental model of evaluating the join predicate on each row of the cross join result then adding back in unmatched rows if an outer join and finally evaluating the where works better for me. The Venn diagrams are good for representing Unions and Intersections and Differences but not joins.
They have some minor educational value for very simple joins, i. Arth - Nope you're wrong. SQL Fiddle sqlfiddle. How did you do these animations? Great answer, the only bit I dislike is your modesty in saying that the Venn diagrams don't do it for you.
The reality is that they are insufficient to model what's going on and this is important to tell, lest people get the wrong idea. Show 13 more comments. For eg- Lets consider Employee and Location table: Inner Join:- Inner join creates a new result table by combining column values of two tables Employee and Location based upon the join-predicate.
The Venn diagrams are mislabelled. Also most of this language is poor. Eg: "When the join-predicate is satisfied by matching non-NULL values, column values for each matched pair of rows of Employee and Location are combined into a result row. Values in rows don't matter other than whether the condition as a whole being true or false. Some values could well be NULL for a true condition. Use images only for what cannot be expressed as text or to augment text.
Inner Join Retrieve the matched rows only, that is, A intersect B. Advisors A ON S. Tushar Gupta - curioustushar Tushar Gupta - curioustushar What is the name of tool?
I find it is interesting as it shows number of rows and venn-diagrams — Grijesh Chauhan. GrijeshChauhan Yeah But you can Try to run it using wine. I used SQLyog using wine.. Show 1 more comment. In simple words: An inner join retrieve the matched rows only.
It doesn't matter if there is a match or not. Peter Mortensen 29k 21 21 gold badges 97 97 silver badges bronze badges. PS This answer is unclear about rows in input vs output.
Inner joins require that a record with a related ID exist in the joined table. Outer joins will return records for the left side even if nothing exists for the right side. For instance, you have an Orders and an OrderDetails table. They are related by an "OrderID". OrderID, Orders. SergeyUr 1 1 gold badge 5 5 silver badges 19 19 bronze badges. Brian Boatright Brian Boatright I appreciate the simple yet realistic example. As you work through the following lessons about outer joins, it might be helpful to refer to this JOIN visualization by Patrik Spathon.
The data for the following lessons was pulled from Crunchbase , a crowdsourced index of startups, founders, investors, and the activities of all three. It was collected Feb. The first table lists a large portion of companies in the database; one row per company. The permalink field is a unique identifier for each row, and also shows the web address. The fields with "funding" in the name have to do with how much outside investment in USD each company has taken on.
The rest of the fields are self-explanatory. The second table lists acquisitions—one row per acquisition. A full outer join, or full join, which is not supported by the popular MySQL database management system, combines and returns all data from two or more tables, regardless of whether there is shared information. Think of a full join as simply duplicating all the specified information, but in one table, rather than multiple tables.
Where matching data is missing, nulls will be produced. These are just the basics, but many things can be done with joins. There are even joins that can exclude other joins! This video explains the difference between various types of joins. It is cued up to begin at the point where the discussion about joins begins.
Share this comparison:. If you read this far, you should follow us:. Diffen LLC, n. Inner Join An inner join focuses on the commonality between two tables. Example of Inner Join Let's consider a common scenario of two tables: product prices and quantities.
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