LEAD() Window Function – with examples in PostgreSQL

Having been forewarned in several of my previous blog posts that I am on a SQL Window Function kick, it should be no surprise that here I am again; sharing what I am studying and learning. I continue to find these powerful constructs highly interesting, to say the least. In this post, I visit the LEAD() Window Function with examples in PostgreSQL. Read on and let’s learn something together…

Note: All data, names or naming found within the database presented in this post, are strictly used for practice, learning, instruction, and testing purposes. It by no means depicts actual data belonging to or being used by any party or organization.

• Xubuntu Linux 18.04.2 LTS (Bionic Beaver)
• PostgreSQL 11.2

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I’ll use a couple of tables from the PostgreSQL practice DVD Rental database for the example queries below.

Throughout this post, let’s assume we have this CTE that returns the below query results:

What does the LEAD() function do?

For a better understanding, let’s first know of its syntax structure:

Syntax observations:

• Accepts at most, 3 parameters: 1 which is mandatory, while the other 2 (denoted in the syntax above by square brackets) are optional.
• offset value parameter – Dictates the number of rows to lead (or access) with the current row. 1 is the default.
• default value parameter – Indicates what will be returned should no more rows be available in the results set according to the offset parameter value. NULL is the default.
• ORDER BY clause is required in the OVER() clause.

In a nutshell, LEAD() accesses current row and subsequent row data. The accessed row beyond the current row depends upon the (2nd) offset value parameter, as we will see with examples that follow.

Prior to reviewing any query results, I’ll set NULL to something a bit more visible for the psql session:

Here is a base query without supplying both the 2nd and 3rd LEAD() function parameters:

The above query is basically mirrored with the next one, in which I specify the defaults for the 2nd and 3rd parameters:

So just what is going on with this LEAD() function? Let’ dissect the below screen shot:

Notice the first row’s ‘leading’ column value of 151.69 is the ‘total_spent’ column value for the next row in the result set. Proceeding from there, the 2nd row’s ‘leading’ column value of 151.73 is in fact, the 3rd rows’ ‘total_spent’ column value.

This pattern continues for all remaining rows in the results set.

All that being said, LEAD() returns the 1st parameter value (‘total_spent’ column in this particular query) for the subsequent row -established by the 2nd function parameter- along with the current row. What subsequent row data is returned when all rows from the query results have been exhausted?

See the below screen shot:

Seeing as there are no remaining rows beyond this last row, !#! is returned. (Which is NULL for this psql session.)

Next, I will change up the parameter values for LEAD() and explore those results:

Even though the image only indicates a couple of rows’ ‘leading’ and corresponding ‘total_spent’ column values, all rows in the result set follow this same pattern.

Notice what happens to the ‘leading’ column value at the 4th row from the end of the results. Since for each of those remaining 4 rows, there is not a ‘total_spent’ value for LEAD() to access, they are indicated with the marker I set for NULl (which is the default).

Perhaps you pose the question of how to access the current and a previous row? Well, you are in luck! There is a LAG() Window Function capable of those type of queries. However, that will be another blog post in the future so be sure and check back when that post drops!

Like what you have read? See anything incorrect? Please comment below and thanks for reading!!!

Explore the official PostgreSQL 11 On-line Documentation for more information.

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Josh Otwell has a passion to study and grow as a SQL Developer and blogger. Other favorite activities find him with his nose buried in a good book, article, or the Linux command line. Among those, he shares a love of tabletop RPG games, reading fantasy novels, and spending time with his wife and two daughters.

Disclaimer: The examples presented in this post are hypothetical ideas of how to achieve similar types of results. They are not the utmost best solution(s). The majority, if not all, of the examples provided, is performed on a personal development/learning workstation-environment and should not be considered production quality or ready. Your particular goals and needs may vary. Use those practices that best benefit your needs and goals. Opinions are my own.