Understanding SQL Joins – All You Need To Know About SQL Joins
Mar 27, · Answer: A self-join can be an inner join or an outer join or even a cross join. A table is joined to itself based upon a column that have duplicate data in different rows. Question: What is a practical use of the Self Join in the real world? Answer: The best example of self join in the real world is when we have a table with Employee data and each row contains information about employee and his/her manager. . A self join is a regular join, but the table is joined with itself.
Suppose we have the following table expllain that is called employee. Now, suppose we want to find out which employees are from the same location as the employee named Joe. In this example, that location would be New York. So, instead of a query like that what we could do is write a nested SQL query exaample a query within another query — which is more commonly called a subquery like this:.
Using a subquery for such a simple question is inefficient. Is there a more efficient and elegant solution to this problem? It turns out that there is a more efficient solution — we can use something called a self join. A self join is basically how to feed your puppy a table is joined to itself.
The way you should visualize a self join for a given table is by imagining that a join is performed between two identical copies of that table. Before we come up with a solution for this problem using a self join, we should go over some concepts so that you can fully understand how a self join works. This will also make the SQL in our self join tutorial a lot easier to understand, which you will see further below.
In a self join we are joining the same table to itself by essentially creating two copies of how to find out the publisher of a website table. But, how do we distinguish between the two different copies of the table — because there is only one table name after all? Well, when we do a self join, the jooin names absolutely must use aliases otherwise the column names would be ambiguous.
Because we need two copies of the employee table, we will just use the aliases e1 and e2 for the employee table when we do a self join. That condition will be our join predicate. If you need a refresher on join predicates or just joins what is a self join explain it with an example general then check this link out: Inner vs.
Outer ih. This will help illustrate how exactly a self join works. The key question that we must ask ourselves is what should our join predicate be in this example? Well, we want to find all the employees who have the same location as Joe.
But is that enough to give us what we want? Well, how to reset a computer without disk we can do is simply add a condition on one of the tables e2 in our example so that wiith only returns the row where the name is Joe.
Generally, queries that refer to the same table can be greatly simplified by re-writing the queries as self joins. And, there is definitely a performance benefit for this as well. It will help tremendously to actually visualize the actual results of a self join internally.
Remember that a self join is just like any other join, where the two tables are merged into one temporary table. First off, you should visualize that we have two separate copies of the employee table, which are given aliases of e1 and e2. And the final results of running the self join query above — the actual what lawn fertilizer to use table — would look like this:.
Are self joins and inner joins the same? You might be wondering if all self joins are also inner joins. After all, in our example above our self join uses an inner join because only the rows that match based on the join predicate are returned — non-matching rows are not returned.
Well, it turns out that a self join and inner join whah completely different concepts. A self join could just as well be an outer join or an inner join — it just depends on how the query what is a self join explain it with an example written. So, we use an implied inner join instead because that gives us the correct results.
The most commonly used example q self joins is the classic employee manager table. The table is called Employee, but holds all employees — including their what is a self join explain it with an example. Every employee has an ID, and there is also a column for the manager ID. Well, it turns out we can use a self join to do this. Try to come up with the SQL on your own before reading our answer.
In order to come up with a correct answer for this problem, our goal should be to perform a self join that will have both the employee information and manager information in one row.
What should our join predicate be in that case? An illustration will help clarify this. Suppose we use that predicate and just select everything after we join the tables. So, our SQL would look like this:. Now, remember that we only want to return the names of the employee and corresponding manager as a pair. So, we can fine-tune the SQL as follows:.
And that is the answer to the employee manager problem using a self join! Feel free to post any comments. Job Hunting? Subscribe to our newsletter for more free interview questions. Follow Wiyh. Find a Job. What is a self join? Explain it with an example and tutorial. Follow programmerintvw.
Mar 15, · A self-join is a Structured Query Language (SQL) statement in which a table is joined with itself, an inner join is performed on a single table with itself, particularly in cases when comparisons have to be made between the records of the same table to determine a relationship or in the cases when the table has a FOREIGN KEY which references its own PRIMARY. The most commonly used example for self joins is the classic employee manager table. The table is called Employee, but holds all employees – including their managers. Every employee has an ID, and there is also a column for the manager ID. So, for example, let’s say we have a table that looks like this – and we call it Employee. The SQL SELF JOIN is used to join a table to itself as if the table were two tables; temporarily renaming at least one table in the SQL statement. Syntax. The basic syntax of SELF JOIN is as follows ?. SELECT cheapautoinsurancewi.info_name, cheapautoinsurancewi.info_name FROM table1 a, table1 b WHERE cheapautoinsurancewi.info_field = cheapautoinsurancewi.info_field;.
Summary : in this tutorial, you will learn how to use the SQL Server self join to join a table to itself. A self join allows you to join a table to itself. It is useful for querying hierarchical data or comparing rows within the same table. A self join uses the inner join or left join clause.
Because the query that uses self join references the same table, the table alias is used to assign different names to the same table within the query. The query references the table T twice. The table aliases t1 and t2 are used to assign the T table different names in the query. Consider the following staffs table from the sample database :. The staffs table stores the staff information such as id, first name, last name, and email. In this example, we referenced to the staffs table twice: one as e for the employees and the other as m for the managers.
The join predicate matches employee and manager relationship using the values in the e. In this tutorial, you have learned how to use an SQL Server self join to query hierarchical data and compare rows in the same table. Note that referencing the same table more than one in a query without using table aliases will result in an error. Up Next.