Performance comparison of SQL JOIN vs. IN clause

Developers must consider performance impact when choosing to retrieve data using JOIN or IN clauses. These two operators have their own advantages and disadvantages, depending on the database server used.

MSSQL considerations

In MSSQL, the performance difference between JOIN and IN mainly depends on the uniqueness of the join column. If the join column is declared UNIQUE, JOIN and IN produce the same execution plan, thus performing similarly.

However, if the join column is not UNIQUE and is not marked as UNIQUE, the IN clause performs better than JOIN. This is because the IN clause uses a more efficient way to handle duplicate values, while the JOIN operator must perform a full table scan to retrieve matching rows.

General considerations

Note that the IN clause and JOIN are not always interchangeable. The JOIN clause performs an equal value join, matching rows based on the equality of the join columns. On the other hand, the IN clause matches rows based on membership in a subquery or scalar expression.

If the data set to be connected is large, the IN clause will be less efficient due to the need to perform multiple subquery evaluations. In this case, JOIN is a better choice because it allows the database server to apply indexing and other optimizations during query execution.

in conclusion

For MSSQL, the choice between JOIN and IN depends on whether the join column is UNIQUE. If it is UNIQUE, both operators provide comparable performance. If not UNIQUE, IN performs better for non-unique values. In either case, it is important to understand the underlying query semantics and potential performance impacts to make informed decisions.