Most RDBMSs have functions that make it easy to calculate the median value from a column in our queries.
Below are three SQL functions we can use to calculate the median value in a column.
Continue readingTag: window functions
Understanding the NTH_VALUE() Function in SQL
Some SQL databases have a window function called NTH_VALUE() that allows us to get a value from a given row in the window frame, based on the row number.
More specifically, the function returns the value of a given expression from the from the N-th row of the window frame, where N is a number that we specify when calling the function.
Continue readingUnderstanding the LAST_VALUE() Function in SQL
In SQL databases, LAST_VALUE() is a window function that returns the last value in an ordered set of values. It allows us to get a value from the last row of a query result set or partition.
You may need to explicitly set the window frame if you want LAST_VALUE() to return the actual last value from the partition or result set. That’s because in many/most DBMSs, the default window frame ends with the current row.
Understanding the FIRST_VALUE() Function in SQL
In SQL databases, the FIRST_VALUE() function is a window function that returns the first value in an ordered set of values.
The exact syntax will probably depend on your DBMS, but it will usually require an OVER clause to determine how the rowset is partitioned and ordered before the window function is applied.
Fix Error “The function ‘ROW_NUMBER’ must have an OVER clause” in SQL Server
If you’re getting an error that reads “The function ‘ROW_NUMBER’ must have an OVER clause” in SQL Server, it’s probably because you’re calling the ROW_NUMBER() function without an OVER clause.
Window functions such as ROW_NUMBER() require an OVER clause (and that clause must have an ORDER BY clause).
To fix this issue, add an OVER clause when calling the ROW_NUMBER() function.
Understanding the DENSE_RANK() Function in SQL
Many relational database management systems (RDBMSs) provide a DENSE_RANK() function that we can use in our SQL queries. The SQL DENSE_RANK() function is a window function that returns the rank of the current row within its partition, without gaps.
The “without gaps” part is what distinguishes the DENSE_RANK() function from the RANK() function.
DENSE_RANK() returns contiguous rank numbers whenever there are ties, whereas RANK() will leave a gap between the tie and the next rank, resulting in noncontiguous rank numbers.
Create a Named Window for a Window Function in SQL
When we create a window function in SQL, we have the option of defining it directly in the OVER clause or in a named window. When we define it in a named window, we can refer to that window name in our OVER clause, which saves us from having to define it directly in the OVER clause.
Understanding the RANK() Function in SQL
In SQL databases, the RANK() function is a window function that returns the rank of the current row within its partition, with gaps.
By “with gaps” I mean that if there are any ties for a given rank, there will be a gap between that rank value and the next rank value.
If you don’t want such gaps, use the DENSE_RANK() function instead, as it returns the rank without gaps.
Understanding the CUME_DIST() Function in SQL
Many relational database management systems (RDBMSs) provide us with a range of window functions.
The CUME_DIST() function is a window function that’s commonly implemented in RDBMSs for the purpose of calculating the cumulative distribution across a data set. In other words, it calculates the relative position of a specified value in a group of values.
Fix Error “The function ‘DENSE_RANK’ must have an OVER clause with ORDER BY” in SQL Server
If you’re getting an error that reads “The function ‘DENSE_RANK’ must have an OVER clause with ORDER BY” in SQL Server, it’s probably because you’re calling the DENSE_RANK() function without an ORDER BY clause.
Window functions such as DENSE_RANK() require an OVER clause, and that clause must have an ORDER BY clause. If you’re getting the above error, it’s likely that you’re providing an OVER clause, but you’re omitting the ORDER BY clause.
To fix this error, add an ORDER BY clause to the OVER clause.