2/23/2023 0 Comments Gideros remove from table![]() In MySQL, or other databases, your query may look like this: SELECT COUNT(*) It shows there are 220 duplicate records. (PARTITION BY first_name, last_name, address ![]() For the purposes of these examples, I’ll check the COUNT of the records about to be deleted, by replacing the DELETE with a SELECT COUNT(*). It’s a good idea to check the records you’re deleting first, by running a SELECT statement using this criteria. (The AskTOM thread uses “WHERE dup 1” but it achieves the same thing). The ROWIDs are then returned to the DELETE statement at the top, which only deletes records where the ROW_NUMBER function (which has an alias of “dup” in this example) are greater than one. The second occurrence of all of those fields will get a number of 2, and so on. This means that the first occurrence of those fields will get the number of 1. It uses the PARTITION BY to create partitions or groups based on the fields I’ve mentioned in the PARTITION BY (which are first_name, last_name, and created_date). The ROW_NUMBER function here is used as an analytic function. (PARTITION BY unique_columns ORDER BY ROWID) dup The query looks like this: DELETE FROM table a Find the ROWID values that are identified as duplicates.Using a subquery to find each ROWID (which is a unique number given to each row in an Oracle table) and the ROW_NUMBER function to find a sequential number for that row, grouped by the fields you specify as unique.It’s been recommended in several places such as StackOverflow questions and an AskTOM thread. The first method I’ll show you is using an analytic function called ROW_NUMBER. But, if you try one method and it has poor performance for your data, try one of the other methods.ĭatabase: Oracle, MySQL, SQL Server, PostgreSQL So, I guess my point is that you might not get the same performance as I do. Several other factors may impact performance such as query cache or background processes on my computer.Your database version and specs will be different. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |