11/5/2023 0 Comments Rank over partition by mysqlI wanted to focus on Paul's original question (that was my problem as well) so I summarize my solution as a working example.īeacuse we want to partition over two column I would create a SET variable during the iteration to identify if a new group was started. In this simple example I only put one but you can have several "PARTITION BY" parts CASE WHEN = part1 AND = part2 THEN ELSE END AS iĪlso a bit late but today I had the same need so I did search on Google and finally a simple general approach found here in Pinal Dave's article If you have mysql compute those column in another order, it might not work. The order of the "computed column" matters.Thus there's already a clear limitation: you can't have several ROW_NUMBER "emulation" of this form at the same time. The ORDER BY clause must reflect your ROW_NUMBER need.I don't see any simple answer covering the "PARTITION BY" part so here's mine : SELECT It may be wise to cast the nulls that create to the actual types of your columns, in the subquery that creates the variables, viz: select := CAST(null as INT), := CAST(null as DATE) This doesn't seem to happen in my experience but I'll gladly accept comments and propose solution if it could reasonably occur I haven't delved into it beyond testing that the method works, but if there is a risk that the predicates in the second WHEN will be optimised away (anything compared to null is null/false so why bother running the assignment) and not executed, it also stops. You can't thus have your rownumber ordered according to one column but your result set ordered to another You might be able to resolve this with subqueries but I believe the docs also state that subquery ordering may be ignored unless LIMIT is used and this could impact performance It's a compromise that the result set has to be ordered by the partition columns too, for the previous column compare to work out. Without them you'll assign null to and things stop working The brackets around := colX) = null are important. The p in pcol means "partition", the o in ocol means "order" - in the general form I dropped the "prev" from the variable name to reduce visual clutter WHEN := pcol1) = null OR := col2) = null OR. ROW_NUMBER() OVER(PARTITION BY pcol1, pcol2. We this get to a situation where this: SELECT But at least evaluating it did its job of keeping the value of col from this row, so it can be evaluated against the next row's col valueīecause the second WHEN is false, it means in situations where the column we are partitioning by (col) has changed, it is the ELSE that gives a new value for restarting the numbering from 1 Because the assignment is made and then the result of the assignment is compared with null, and anything equated with null is false, this predicate is always false. The second WHEN predicate is always false, and it exists purely to assign a new value to Because this row's col is different to the previous row's col (we know this because if it were the same, the first WHEN would have been used), we have to assign the new value to keep it for testing next time. This causes the second WHEN to be evaluated. This first predicate also returns false every time col changes (current row is different to previous row). This return led values is stored in It's a feature of MySQL that assignment returns the new value of what is assigned into into the result rows.įor the first row on the result set, is null (it is initialised to null in the subquery) so this predicate is false. If this row's col is the same as the previous row's col then is incremented and returned from the CASE. Enter CASE WHEN: := CASEĪs outline ld, order of assignment of prevcol is important - prevcol has to be compared to the current row's value before we assign it a value from the current row (otherwise it would be the current rows col value, not the previous row's col value). So we need to use something with a guaranteed order of execution. If these are ever evaluated bottom up, the row number will stop working (no partitions) (row number variable that uses partition variable), Don't assign variables in one SELECT item and use them in another oneĪs such, there's a risk they will churn out the wrong answer, because they typically do a select The answers here that use in-query variables mostly/all seem to ignore the fact that the documentation says (paraphrase):ĭon't rely on items in the SELECT list being evaluated in order from top to bottom. Important: Please consider upgrading to MySQL 8+ and use the defined and documented ROW_NUMBER() function, and ditch old hacks tied to a feature limited ancient version of MySQL
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