As, MySQL optimizer may come into place and change our presumed order of evaluation. General observation is that using ORDER BY with evaluation of the user variables in the same query block, does not ensure that the values will be correct always. Note that, the first and last rank numbers assigned by the MySQL RANK () function are always 1 and the number of rows in the partition. If no partition is created, it will consider the whole table as a single partition and sort the records. Supported in MySQL 8.0 for backward compatibility but is subject toĪlso, thanks to a fellow SO member, came across this blog by MySQL Team: The ORDER BY clause sorts the records in the partitions. User variable in statements other than SET. Previous releases of MySQL made it possible to assign a value to a | - | - | - | - |Īn interesting note from MySQL Docs, which I discovered recently: Result | user_game_rank | user_id | game_detail | game_id | WHEN := dt.user_id /* Notice := instead of = */ We will need to evaluate row number and assign the user_id value to variable within the same expression. For example, there is no guarantee that SELECT first and then performs the assignment. The order of evaluation for expressions involving user variables is Eventually, we will assign row number accordingly.Įdit: Based on MySQL docs and Linoff's observation: We would store the previous row values in the User-defined variables, and then check the current row's value(s) against the previous row. It will be like a Looping technique (which we use in application code, eg: PHP). Now, we use this result-set and use conditional CASE.WHEN expressions to evaluate the row numbering. In a Derived Table (subquery inside the FROM clause), we order our data such that all the rows having same user_id values come together, with further sorting between them based on game_detail in Descending order.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |