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.
In 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.
If you’re getting error 3579 in MySQL, which reads something like “Window name ‘wf3’ is not defined“, it’s probably because you’re referring to a named window that doesn’t exist.
To fix this issue, make sure you refer to a named window that exists.
We 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”.
In MySQL, we can use the sys.sys_get_config() function to get a configuration option value from the sys_config table.
If you’re getting error 3580 in MySQL, which reads “There is a circularity in the window dependency graph“, it’s probably because you’re using named windows that refer to each other in a way that creates a circular reference.
To fix this issue, make sure you don’t create a circular reference when referring to multiple named windows.
In 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.
When we create or alter a CHECK constraint in MySQL, we have the option of setting it to ENFORCED or NOT ENFORCED. This determines whether or not MySQL will enforce the rules of the constraint whenever data that violates the constraint tries to enter the database.
We can use the ALTER TABLE statement to change the enforcement state of an existing CHECK constraint in MySQL, as shown in the following example.