In MySQL, RANK() is a window function that returns the rank of the current row within its partition, with gaps.
By “gaps” this means that it returns noncontiguous rank numbers whenever there are peers. Peers are considered ties and receive the same rank, but in such cases, we get a gap between this rank value and the next rank value.
Continue readingIn MySQL, PERCENT_RANK() is a window function that returns the percentage of partition values less than the value in the current row, excluding the highest value.
We can use PERCENT_RANK() to evaluate the relative standing of a value within a query result set or partition. Return values range from 0 to 1.
In MySQL, CUME_DIST() is a window function that returns the cumulative distribution of a value within a group of values. This is the percentage of partition values less than or equal to the value in the current row. The return values range from 0 to 1.
SQL provides us with an easy way to automatically compute the moving average of a given column.
The moving average (also known as the rolling average, running average, moving mean (MM), or rolling mean) is a series of averages of different selections of the full data set. The moving average changes as we move through the data set. We can add the moving average to our SQL query result sets to see how it changes across the result set.
Continue readingWe can use SQL to create a running total of a given column. By this I mean, we can create a column that calculates the cumulative sum of a given column.
The running total/cumulative sum is the sum of all rows up until the current row. The result is that the sum increases (or decreases in the case of negative values) with each row returned in the result set. Each row’s value is added to the cumulative amount from all prior rows, so for any given row, we get the total sum for all rows up to that point – the “running total”.
Continue readingIn SQL, we can use the following method to compute the number of items within a given range when using a window function.
By this I mean we can use a frame clause in the window function to output how many items have a value that’s in a similar range to the value in the current row.
Continue readingIf you’re getting MySQL error 1140, which reads something like “In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column ‘world.City.District’; this is incompatible with sql_mode=only_full_group_by“, it could be that you need to introduce a window function to the query.
This error can happen when we want multiple rows to contain aggregate values. For example, we might want to return subtotals of all rows within a group of rows. A window function can help us achieve this outcome.
Continue readingWhen we use an OVER() clause to create a window function in SQL, we often use a PARTITION BY clause to partition the results. This can be handy if we want to do stuff like calculate subtotals.
But we can also use an empty OVER clause to calculate a grand total.
We might want to do this if we’re already using another OVER clause to calculate subtotals, but we also want a column to provide the grand total.
We can use the OVER() clause to create window functions in our SQL queries. A window function can be a useful tool that allows us to do things like compute moving averages, rank items, calculate cumulative sums, and much more.
Another common task for a window function might be to compute subtotals.
Continue readingIf you’re getting MySQL error 3582, which reads something like “Window ‘w1’ has a frame definition, so cannot be referenced by another window“, it’s probably because you’re referencing a named window that has a frame clause.
This can happen when we use the WINDOW clause to define a named window for a window function, and we include a frame clause in that window. When we then try to reference that window, MySQL tells us that we can’t, due to the frame clause.