Three table JOIN Example SQL
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Joining three tables in single SQL query can be very tricky if you are not good with the concept of SQL Join. SQL Joins have always been tricky not only for new programmers but for many others, who are in programming and SQL for more than 2 to 3 years. There are enough to confuse someone on SQL JOIN ranging from various types of SQL JOIN like INNER and OUTER join, LEFT and RIGHT outer join, CROSS join etc. Between all of these fundamentals, What is most important about Join is, combining multiple tables. If you need data from multiple tables in one SELECT query you need to use either subquery or JOIN. Most of the times we only join two tables like Employee and Department but sometimes you may require joining more than two tables and a popular case is joining three tables in SQL.
In the case of joining three tables table, 1 relates to table 2 and then table 2 relates to table 3. If you look at closely you find that table 2 is a joining table which contains primary key from both table 1 and table 2. As I said it can be extremely confusing to understand join of three or more tables.
I have found that understanding table relationship as the primary key and foreign key helps to alleviate confusion than the classical matching row paradigm.
SQL Join is also a very popular topic in SQL interviews and there are always been some questions from Joins, like the difference between INNER and OUTER JOIN, SQL query with JOIN e.g. Employee Department relationship and Difference between LEFT and RIGHT OUTER JOIN etc. In short this is one of the most important topics in SQL both from experience and interview point of view.
In the case of joining three tables table, 1 relates to table 2 and then table 2 relates to table 3. If you look at closely you find that table 2 is a joining table which contains primary key from both table 1 and table 2. As I said it can be extremely confusing to understand join of three or more tables.
I have found that understanding table relationship as the primary key and foreign key helps to alleviate confusion than the classical matching row paradigm.
SQL Join is also a very popular topic in SQL interviews and there are always been some questions from Joins, like the difference between INNER and OUTER JOIN, SQL query with JOIN e.g. Employee Department relationship and Difference between LEFT and RIGHT OUTER JOIN etc. In short this is one of the most important topics in SQL both from experience and interview point of view.
