SQL tips and tricks

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

Feel free to contribute your own by opening a pull requests!

Table of contents

• Use a leading comma to seperate fields
• Use a dummy value in the WHERE clause
• Ident your code where appropriate
• Be aware of how NOT IN behaves with NULL values
• Rename calculated fields to avoid ambiguity

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
;

Ident 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
;

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