Key Differences Between Subqueries and Joins in SQL

1. What They Are:

   • Subqueries: These are like mini-queries inside a bigger SQL query. They give you temporary results that can help filter or set conditions.
   • Joins: These connect rows from two or more tables by using matching columns. This makes it easier to compare data from different tables.

2. Speed:

   • Studies show that joins are usually faster than subqueries, especially when dealing with large amounts of data. For example, SQL join operations can be about 30% faster than subqueries when the queries get complicated.

3. Ease of Reading:

   • Subqueries might make it easier to read when you need to filter data in steps. This works well for simple data sets, but they can be confusing if they get too nested.
   • Joins, on the other hand, show relationships more clearly and are usually easier for the database to handle.

4. When to Use Them:

   • Use subqueries when you want to check if something exists or to calculate totals without needing all the records.
   • Use joins when you want to pull together complete data from related tables, like getting user info from a 'Users' table and their purchase records from a 'Purchases' table.

5. Different Types:

   • Subqueries come in different kinds, like scalar, row, column, or table subqueries, based on what they return.
   • Joins include inner joins, outer joins (left and right), and cross joins, giving you different options for how to combine the data.

Knowing these differences is important for building efficient SQL queries, especially when working with databases in school.