Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

When working with databases, it's important to understand how to write efficient and effective queries. One common issue that can arise is when a subquery returns more than one value. This can cause problems in your query and make it difficult to get the results you need. In this blog post, we will explore why this issue occurs and how to avoid it.

Subqueries are a powerful tool for working with databases. They allow you to retrieve data from multiple tables at once and perform complex queries that would be difficult or impossible to write using a single query. However, when used incorrectly, subqueries can cause problems in your database.

One common issue that can occur when using subqueries is when the subquery returns more than one value. This can happen if you are using a subquery as an expression, such as in a WHERE clause or a JOIN condition. For example:

sql
SELECT * FROM orders
WHERE order_date BETWEEN (SELECT start_date FROM events) AND (SELECT end_date FROM events);

In this query, the subquery `(SELECT start_date FROM events)` returns a single value, which is used as the start date for the `BETWEEN` clause. However, if the subquery returned more than one value, the query would return an error or unexpected results.

To avoid this issue, it's important to understand how subqueries work and how they are used in your database. When using a subquery as an expression, you should always make sure that the subquery returns a single value. This can be done by using a `LIMIT` clause or by using a `UNION` statement to combine multiple subqueries into a single result set.

For example:

sql
SELECT * FROM orders
WHERE order_date BETWEEN (SELECT start_date FROM events LIMIT 1) AND (SELECT end_date FROM events LIMIT 1);

In this query, the `LIMIT` clause ensures that the subquery returns a single value, which is used as the start and end dates for the `BETWEEN` clause. This ensures that the query will return only the orders that fall within the specified date range.

Another way to avoid the issue of a subquery returning more than one value is to use a `UNION` statement. A `UNION` statement allows you to combine the results of multiple subqueries into a single result set, which can be used as an expression in your query. For example:

sql
SELECT * FROM orders
WHERE order_date BETWEEN (SELECT start_date FROM events) AND (SELECT end_date FROM events UNION ALL SELECT start_date FROM events);

In this query, the `UNION ALL` statement combines the results of two subqueries into a single result set. The first subquery returns the start and end dates for the event, while the second subquery returns the start date for another event. This ensures that the query will return only the orders that fall within the specified date range.

It's important to note that when using `UNION` statements, you should always use the `ALL` keyword. The `ALL` keyword ensures that all rows returned by the subqueries are included in the final result set, even if they have duplicate values. Without the `ALL` keyword, only unique rows will be returned.

In addition to using `LIMIT` clauses and `UNION` statements, there are other ways to avoid the issue of a subquery returning more than one value. One way is to use a `LEFT JOIN` instead of an `INNER JOIN`. A `LEFT JOIN` will return all rows from the left table, even if there are no matching rows in the right table. This can be useful when you want to include all orders, even if they don't match any events in the database.

For example:

sql
SELECT * FROM orders
LEFT JOIN events ON order_date BETWEEN start_date AND end_date;

In this query, the `LEFT JOIN` ensures that all orders are included