SQL JOIN Basics.
How to we define Joins?
- You can retrieve and manipulate data from more than one table in a single SELECT statement with using JOIN.
- You are going to use columns from multiple tables to pair tables.
- You can join columns with different names as long as the columns you are connecting are join-compatible.
- It is best if join columns have exactly the same data type.
Why Are Joins Necessary?
- It can happen that one table might not give you all the information you need about a particular entity.
- The relational model lets you partition your data using normalization and relies on the join operation to produce comprehensible reports.
- Joins are possible because of the relational model, and they are necessary because of the relational model.
How do we associating Data from Separate Tables?
- You can discover new relationships among data in different tables by joining them.
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Generally there are two ways to connect two or more tables together.
Using the word JOIN between the tables:SQL server, MySQL, Oracle, PostgreSQL :
select First_name "First name", Last_name "Last Name", email, Telephone_type "Phone type", Telephone_number as "Phone number" from customers c JOIN telephones t on (c.customer_id= t.customer_id);
Using comma between the tables:SQL server, MySQL, Oracle, PostgreSQL :select First_name "First name", Last_name "Last Name", email, Telephone_type "Phone type", Telephone_number as "Phone number" from customers c, telephones t where c.customer_id= t.customer_id;
The result should be for both examples:First name Last Name email Phone type Phone number Ricard Nixon Ricard.Nixon@hotmail.com Work (978) 667-94 Ricard Nixon Ricard.Nixon@hotmail.com Home (978) 667-94 Robert Redford R.Redford@google.com Unknown (678) 223-94 Robert Redford R.Redford@google.com Home (679) 234-94 Judith Bowman J.Bowman@imb.com Cell (371) 153-54 Judith Bowman J.Bowman@imb.com Work (371) 143-54 Sandra Emerson Sandra.Emerson@mySql.com Unknown (881) 141-54 Sandra Emerson Sandra.Emerson@mySql.com Unknown Marcy Darnovsky Marcy@oracle.com Unknown (881) 141-54 Marcy Darnovsky Marcy@oracle.com Unknown
Join Provides Flexibility
- If a table must be split into two tables, you can always reconstruct the rows using the join.
- If new and related information appears, you can design tables with appropriate join columns, so the new data can be linked to the existing data.
How to get a Good Join?
- Ideally, a join column is a key column for its table - either as a primary key or as a foreign key.
- When a key is composite, you can join on all the columns of the key.
- Key columns are usually ideal candidate columns on which to construct a join.
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Rules for a join is to have meaningful results:
- Tables must be named in the table list in order for the system to perform the join.
- The columns in the join must have related meaning.
- Columns that participate in a JOIN does not need to have the same name, although they often will.
- Beware of that columns with NULLs will never join because NULLs represent unknown or inapplicable values.
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Performance is often an issue with joins. Here are some performance guidelines:
- Join columns should be indexed.
- Join columns should have exactly the same datatypes.
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