SQL Joins: What are INNER, LEFT, RIGHT, and FULL JOINs and How Do They Differ?

Understanding SQL Joins: INNER, LEFT, RIGHT, and FULL JOINs

In SQL, JOIN clauses are instrumental in combining rows from multiple tables based on a common field. There are various JOIN types, each catering to specific scenarios.

INNER JOIN

INNER JOIN intertwines rows where a common field in both tables exhibits matching values. For instance, if the TableA and TableB fields both contain an "id" column, the JOIN would retrieve rows where TableA.id equals TableB.id.

LEFT JOIN

As the name implies, LEFT JOIN prioritizes rows from the leftmost table (TableA in our example). In any instance of a field match within TableB, the row is included in the result. Otherwise, missing data is represented as NULLs.

RIGHT JOIN

Inversely, RIGHT JOIN focalizes on rows from the rightmost table (TableB in our case). Rows are drawn from TableB while also retrieving matching data from TableA. If no match exists, NULLs are used in the result.

FULL JOIN

FULL JOINs unite the power of both LEFT and RIGHT JOINs, ensuring the inclusion of all rows from TableA and TableB. If a row lacks a match in one table, it will appear with NULLs in its corresponding fields.

Impact of Join Order

It's worth noting that join order significance hinges on the type of JOIN employed. INNER JOINs are impartial to order, while ordering affects the outcomes of OUTER JOINs (LEFT, RIGHT, and FULL).

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