SQL tips and tricks

A (somewhat opinionated) list of SQL tips and tricks I've picked up over the years.

Please note that some of these tips might not be relevant for all RDBMs. For example, the :: syntax (tip 5) does not work in SQLite.

Table of contents

Formatting/readability

Use a leading comma to separate fields

Use a leading comma to seperate fields in the SELECT clause rather than a trailing comma.

Clearly defines that this is a new column vs code that's wrapped to multiple lines.
Visual cue to easily identify if the comma is missing or not. Varying line lengths makes it harder to determine.

SELECT
employee_id
, employee_name
, job
, salary
FROM employees
;

Also use a leading AND in the WHERE clause, for the same reasons (following tip demonstrates this).

Use a dummy value in the WHERE clause

Use a dummy value in the WHERE clause so you can dynamically add and remove conditions with ease:

SELECT *
FROM employees
WHERE 1=1 -- Dummy value.
AND job IN ('Clerk', 'Manager')
AND dept_no != 5
;

Indent your code where appropriate

Indent your code to make it more readable to colleagues and your future self:

-- Bad:
SELECT 
timeslot_date
, timeslot_channel 
, overnight_fta_share
, IFF(DATEDIFF(DAY, timeslot_date, CURRENT_DATE()) > 7, LAG(overnight_fta_share, 1) OVER (PARTITION BY timeslot_date, timeslot_channel ORDER BY timeslot_activity), NULL) AS C7_fta_share
, IFF(DATEDIFF(DAY, timeslot_date, CURRENT_DATE()) >= 29, LAG(overnight_fta_share, 2) OVER (PARTITION BY timeslot_date, timeslot_channel ORDER BY timeslot_activity), NULL) AS C28_fta_share
FROM timeslot_data
;

-- Good:
SELECT 
timeslot_date
, timeslot_channel 
, overnight_fta_share
, IFF(DATEDIFF(DAY, timeslot_date, CURRENT_DATE()) > 7, -- First argument of IFF.
	LAG(overnight_fta_share, 1) OVER (PARTITION BY timeslot_date, timeslot_channel ORDER BY timeslot_activity), -- Second argument of IFF.
		NULL) AS C7_fta_share -- Third argument of IFF.
, IFF(DATEDIFF(DAY, timeslot_date, CURRENT_DATE()) >= 29, 
		LAG(overnight_fta_share, 2) OVER (PARTITION BY timeslot_date, timeslot_channel ORDER BY timeslot_activity), 
			NULL) AS C28_fta_share
FROM timeslot_data
;

Useful features

You can use the `::` operator to cast the data type of a value

In some RDBMs you can use the :: operator to cast a value from one data type to another:

SELECT CAST('5' AS INTEGER); -- Using the CAST function.
SELECT '5'::INTEGER; -- Using :: syntax.

Anti-joins are your friend

Anti-joins are incredible useful, mostly (in my experience) for when when you only want to return rows/values from one table that aren't present in another table.

You could instead use a subquery although conventional wisdom dictates that anti-joins are faster.
EXCEPT is an interesting operator for removing rows from one table which appear in another query table but I suggest you read up on it further before using it.

-- Anti-join.
SELECT 
video_content.*
FROM video_content
    LEFT JOIN archive
    on video_content.series_id = archive.series_id
WHERE 1=1
AND archive.series_id IS NULL -- Any rows with no match will have a NULL value.

-- Subquery.
SELECT 
*
FROM video_content
WHERE 1=1
AND series_id NOT IN (SELECT DISTINCT SERIES_ID FROM archive) -- Be mindful of NULL values (see tip 9).

-- Correlated subquery.
SELECT 
*
FROM video_content
WHERE 1=1
AND NOT EXISTS (
        SELECT 1
        FROM archive a
        WHERE a.series_id = vc.series_id
    )

-- EXCEPT.
SELECT series_id
FROM video_content
EXCEPT
SELECT series_id
FROM archive

Use QUALIFY to filter window functions

QUALIFY lets you filter the results of a query based on a window function. This is useful for a variety of reasons, including to reduce the number of lines of code needed.

For example, if I want to return the top 10 markets per product I can use QUALIFY rather than an in-line view:

-- Using QUALIFY:
SELECT 
product
, market
, SUM(revenue) as market_revenue 
FROM sales
GROUP BY product, market
QUALIFY DENSE_RANK() OVER (PARTITION BY product ORDER BY SUM(revenue) DESC)  <= 10
ORDER BY product, market_revenue
;

-- Without QUALIFY:
SELECT 
product
, market
, market_revenue 
FROM
(
SELECT 
product
, market
, SUM(revenue) as market_revenue
, DENSE_RANK() OVER (PARTITION BY product ORDER BY SUM(revenue) DESC) AS market_rank
FROM sales
GROUP BY product, market
)
WHERE market_rank  <= 10
ORDER BY product, market_revenue
;

You can (but shouldn't always) `GROUP BY` column position

Rather than use the column name you can GROUP BY or ORDER BY using column position.

For ad-hoc/one-off queries this can be useful but for production code you should always refer to a column by its name.

SELECT 
dept_no
, SUM(salary) as dept_salary
FROM employees
GROUP BY 1 -- dept_no is the first column in the SELECT clause.
ORDER BY 2 DESC
;

Common pitfalls

Be aware of how `NOT IN` behaves with `NULL` values

NOT IN doesn't work if NULL is present in the values being checked against. As NULL represents Unknown the SQL engine can't verify that the value being checked is not present in the list.

Instead use NOT EXISTS.

INSERT INTO departments (id)
VALUES (1), (2), (NULL);

-- Doesn't work due to NULL being present.
SELECT * 
FROM employees 
WHERE department_id NOT IN (SELECT DISTINCT id from departments)

-- Solution.
SELECT * 
FROM employees e
WHERE NOT EXISTS (
    SELECT 1 
    FROM departments d 
    WHERE d.id = e.department_id
)
;

Rename calculated fields to avoiding ambiguity

When creating a calculated field you might be tempted to rename it to an existing column but this can lead to unexpected behaviour, such as a window function operating on the wrong field:

INSERT INTO products (product, revenue)
VALUES 
    ('Shark', 100),
    ('Robot', 150),
    ('Alien', 90);

-- The window function will rank the 'Robot' product as 1 when it should be 3.
SELECT 
product
, CASE product WHEN 'Robot' THEN 0 ELSE revenue END AS revenue
, RANK() OVER (ORDER BY revenue DESC)
FROM products

Always specify which column belongs to which table

When you have complex queries with multiple joins it pays to be able to trace back an issue with a value to its source.

Additionally, your RDBMS might raise an error if two tables share the same column name and you don't specify which column you are using.

SELECT 
vc.video_id
, vc.series_name
, metadata.season
, metadata.episode_number
FROM video_content as vc 
    INNER JOIN video_metadata as metadata
    ON vc.video_id = metadata.video_id

Understand the order of execution

If I had to give one piece of advice to someone learning SQL it'd be to understand the order of execution (of clauses). It will completely change how you write queries. This blog post is a fantastic resource for learning.

Comment your code!

While in the moment you know why you did something if you revisit the code weeks, months or years later you might not remember.

In general you should strive to write comments that explain why you did something, not how.
Your colleagues and future self will thank you!

SELECT 
video_content.*
FROM video_content
    LEFT JOIN archive -- New CMS cannot process archive video formats. 
    on video_content.series_id = archive.series_id
WHERE 1=1
AND archive.series_id IS NULL

Read the documentation (in full)

Using Snowflake I once needed to return the latest date from a list of columns and so I decided to use GREATEST().

What I didn't realise was that if one of the arguments is NULL then the function returns NULL.

If I'd read the documentation in full I'd have known! In many cases it can take just a minute or less to scan the documentation and it will save you the headache of having to work out why something isn't working the way you expected:

-- If I'd read the documentation further I'd also have realised that my solution
--to the NULL problem with GREATEST()...

SELECT COALESCE(GREATEST(signup_date, consumption_date), signup_date, consumption_date)

-- ... could have been solved with the following function:
SELECT GREATEST_IGNORE_NULLS(signup_date, consumption_date)

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