When it comes to databases, both joins and unions are used to combine data from multiple tables, but they serve different purposes and operate in different ways. A join is used to combine rows from two or more tables based on a related column between them. This means that the rows are combined horizontally, allowing you to pull in related data for each entry. For instance, if you have a "Customers" table and an "Orders" table, you can perform a join to get a comprehensive view that shows each customer alongside their corresponding orders by linking the customer ID from both tables.
On the other hand, a union is used to combine the result sets of two or more SELECT queries into a single result set. This operation stacks the results vertically, meaning that the tables involved must have the same number of columns and compatible data types in those columns. For example, if you have two tables, "Employees" and "Contractors," both with fields for "ID" and "Name," you can use a union to create a single view that lists all individuals, regardless of their employment type. While the rows from each table remain distinct, the union eliminates any duplicate rows by default unless specified otherwise with the UNION ALL operator.
In summary, the key difference lies in how they combine data and the structure of the data being combined. Joins create a single row for related entries from different tables, enhancing the richness of the dataset, whereas unions consolidate multiple queries into a single list without maintaining relationships. Understanding these differences is crucial for effectively querying relational databases and ensuring that data is presented in the desired format for analysis or reporting purposes.