What is the difference between Left, Right, Outer and Inner Joins?

Question

I am wondering how to differentiate all these different joins ...

Answer 1

There are three basic types of join:

• INNER join compares two tables and only returns results where a match exists. Records from the 1st table are duplicated when they match multiple results in the 2nd. INNER joins tend to make result sets smaller, but because records can be duplicated this isn't guaranteed.
• CROSS join compares two tables and return every possible combination of rows from both tables. You can get a lot of results from this kind of join that might not even be meaningful, so use with caution.
• OUTER join compares two tables and returns data when a match is available or NULL values otherwise. Like with INNER join, this will duplicate rows in the one table when it matches multiple records in the other table. OUTER joins tend to make result sets larger, because they won't by themselves remove any records from the set. You must also qualify an OUTER join to determine when and where to add the NULL values:
  • LEFT means keep all records from the 1st table no matter what and insert NULL values when the 2nd table doesn't match.
  • RIGHT means the opposite: keep all records from the 2nd table no matter what and insert NULL values whent he 1st table doesn't match.
  • FULL means keep all records from both tables, and insert a NULL value in either table if there is no match.

Often you see will the OUTER keyword omitted from the syntax. Instead it will just be "LEFT JOIN", "RIGHT JOIN", or "FULL JOIN". This is done because INNER and CROSS joins have no meaning with respect to LEFT, RIGHT, or FULL, and so these are sufficient by themselves to unambiguously indicate an OUTER join.

Here is an example of when you might want to use each type:

• INNER: You want to return all records from the "Invoice" table, along with their corresponding "InvoiceLines". This assumes that every valid Invoice will have at least one line.
• OUTER: You want to return all "InvoiceLines" records for a particular Invoice, along with their corresponding "InventoryItem" records. This is a business that also sells service, such that not all InvoiceLines will have an IventoryItem.
• CROSS: You have a digits table with 10 rows, each holding values '0' through '9'. You want to create a date range table to join against, so that you end up with one record for each day within the range. By CROSS-joining this table with itself repeatedly you can create as many consecutive integers as you need (given you start at 10 to 1st power, each join adds 1 to the exponent). Then use the DATEADD() function to add those values to your base date for the range.