Besides mailing lists, other applications for unions include when combining archived data with active data or comparing data in different tables that share a similar structure. This results in a single list of all staff and customers, with a shared data structure. Instead, we simply combine the result sets, knowing that duplicates may exist. Notice that we use UNION ALL instead of UNION to prevent MySQL from eliminating duplicate rows, which can be computationally expensive when we have a large result set. Example Here is an example of a MySQL INNER JOIN: SELECT suppliers.supplierid, suppliers.suppliername, orders.orderdate FROM suppliers INNER JOIN orders ON suppliers.supplierid orders. SELECT first_name, last_name, email_address FROM customers UNION ALL SELECT first_name, last_name, email_address FROM staff You want to combine the results of both tables into a single result set, to produce a mailing list. Each subclause provides a subquery that produces a result set, and associates a name with the subquery. UNIONs are best used when we need to combine two queries that would otherwise be difficult to create in one query.įor example, let's say you have two tables, customers and staff, both with similar column names. To specify common table expressions, use a WITH clause that has one or more comma-separated subclauses. Looking at the above query, you'll see that this could be achieved with a simple SELECT statement. Also, bear in mind that the number of columns in all SELECT statements must be the same, with the same data types, or else you'll get an error. Remove WHERE SP.ServiceFP.Service from outer WHERE clause as again, SP. from outer SELECT as SP and FP are only visible to inner queries, use SELECT instead. Instead, use SELECT FROM (.) SP JOIN (.) FP. () SP LEFT JOIN () SP is not a valid syntax. All SELECT queries ( return ) number of column. Wrap inner SELECT queries into another SELECT query to make JOIN work, e.g. Notice how I used the UNION keyword to combine the two results. MySQL Union query We can combine more than one SELECT query by using UNION command to get single result set. This will output the result set of 1 and 2, as if they were in a single column. Specifically, it takes the values from one table or query, and instead of putting them side-by-side with another table or another query, it puts them together over-under. A UNION query is used to combine the results of two or more SELECT statements into a single result set.
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