![]() This also gives you the added benefit of being able to ORDER BY different columns for each union. (SELECT 3 AS rank, id, add_date from my_table WHERE distance BETWEEN 5 and 15 ORDER BY id LIMIT 9999999999)Ī high LIMIT means that you could add an OFFSET on the overall query if you want to do something such as pagination. (SELECT 2 AS rank, id, add_date FROM my_table WHERE distance BETWEEN 5 AND 15 ORDER BY rank LIMIT 9999999999) To force MySQL to not do this optimisation you can add a LIMIT clause like so: (SELECT 1 AS rank, id, add_date FROM my_table WHERE distance < 5 ORDER BY add_date LIMIT 9999999999) This optimisation causes a problem when you are in a situation where you need to order within the subquery. The last sentence of this is a bit misleading because it should have an effect. Without LIMIT in a SELECT, it is optimized away because it will have The order of those rows in the final UNION result. To retrieve for the SELECT, even though it does not necessarily affect LIMIT, so that it is used to determine the subset of the selected rows The use of ORDER BY in this context is typically in conjunction with Nothing about the order in which the rows appear in the final resultīecause UNION by default produces an unordered set of rows. However, use of ORDER BY for individual SELECT statements implies (SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10) Inside the parentheses that enclose the SELECT: (SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10) UNION To apply ORDER BY or LIMIT to an individual SELECT, place the clause The optimisation is mentioned in the docs and says: ![]() This is because by default a UNION returns an unordered list so therefore an ORDER BY would do nothing. MySQL always uses the name of the column in the first SELECT statement will be the column names of the result set (output). ![]() It comes with a default feature that removes the duplicate rows from the result set. When you use an ORDER BY clause inside of a sub query used in conjunction with a UNION mysql will optimise away the ORDER BY clause. MySQL Union is an operator that allows us to combine two or more results from multiple SELECT queries into a single result set. ![]() Or add an artificial id, in which case this way makes it easy to sort by different parameters in each query, but it otherwise is the same as the accepted answer. The ability to add a limit is simply an extra bonus.Īnd of course feel free to turn the union all into a union and add a sort for the whole query. It also allows you to return the artificial column to the querying program so it has a context it can sort by or organize.īut this way has the advantage of being fast, not introducing extra variables, and making it easy to separate out each query including the sort. However by using the selected answer's way it might become clearer depending on complexity and how related the data is because you are conceptualizing the sort. It keeps the individual queries clearer and allows you to sort by different parameters in each query. Don't forget, union all is a way to add records to a record set without sorting or merging (as opposed to union). ![]()
0 Comments
Leave a Reply. |