Mastering Inner Joins in SQL

SQL Joins

• A Cartesian join in SQL is a join operation that returns the Cartesian product of rows from two or more tables, resulting in a large result set with an excessive number of rows.
• The result of a Cartesian join between two tables with 10 and 20 rows respectively is a result set with 10 x 20 = 200 rows.

Purpose of JOINs

• The purpose of using JOINs in SQL is to combine data from two or more tables based on a related column between them, allowing for more complex data retrieval and analysis.

Types of JOINs

• An equality join is a type of join that combines rows from two tables where the join condition is based on equality of values in a common column.

Common Errors

• The ORA-00918 error in a join operation is caused by a mismatch in the number of columns being selected and the number of values being inserted.

Fully Qualified Names (FQN)

• A fully qualified name (FQN) is a way of specifying a column or table name to avoid ambiguity in SQL statements, especially in join operations.
• In a self-join, a fully qualified name is used to distinguish between columns with the same name from different tables.

ANSI 92 Standard

• The ANSI 92 standard uses the INNER JOIN clause to specify the join condition, followed by the ON clause to specify the join predicate.

Join Optimization

• A rule of thumb for the number of INNER JOIN/ON combinations in a join with 4 tables is to minimize the number of combinations to reduce computational complexity.

Self-Joins

• In a self-join, cases where a value would be equal to itself can be eliminated by using a predicate in the join condition, such as "WHERE t1.column != t2.column".
• The purpose of a self-join is to combine rows from the same table based on a common column, allowing for hierarchical or recursive data structures.
• The purpose of the FQN in a self-join is to distinguish between columns with the same name from different aliases of the same table.