A simple example of a nested SQL query

Originally published at https://rivea0.github.io/blog.

Let's say we have this table:

city	country	population	latitude	longitude
Guadalajara	Mexico	1500800	20.659699	-103.349609
Toronto	Canada	2795060	43.653226	-79.383184
Houston	United States	2195914	29.760427	-95.369803
New York	United States	8405837	40.712784	-74.005941
Philadelphia	United States	1553165	39.952584	-75.165222
Havana	Cuba	2106146	23.05407	-82.345189
Mexico City	Mexico	8555500	19.432608	-99.133208
Phoenix	United States	1513367	33.448377	-112.074037
Los Angeles	United States	3884307	34.052234	-118.243685
Ecatepec de Morelos	Mexico	1742000	19.601841	-99.050674
Montreal	Canada	1717767	45.501689	-73.567256
Chicago	United States	2718782	41.878114	-87.629798

We want to get all the cities west of Chicago.
As we go towards the west, the longitude must be less than the longitude of Chicago — the west of the prime meridian has longitudes that are negative numbers.

Here is the pseudocode:

SELECT * FROM north_american_cities
WHERE longitude < ${Chicago's longitude}

So, what we need to do is somehow get the value of Chicago's longitude and use it as comparison.
It is by itself easy:

SELECT longitude FROM north_american_cities
WHERE city = 'Chicago';

What we can do is simply put it inside the parenthesis and use it as a value itself, just like a variable:

SELECT * FROM north_american_cities
WHERE longitude < (
    SELECT longitude
    FROM north_american_cities
    WHERE city = 'Chicago'
);

The result is this:

city	country	population	latitude	longitude
Guadalajara	Mexico	1500800	20.659699	-103.349609
Houston	United States	2195914	29.760427	-95.369803
Mexico City	Mexico	8555500	19.432608	-99.133208
Phoenix	United States	1513367	33.448377	-112.074037
Los Angeles	United States	3884307	34.052234	-118.243685
Ecatepec de Morelos	Mexico	1742000	19.601841	-99.050674

Furthermore, we can order the longitudes from west to east using the ORDER BY clause:

SELECT * FROM north_american_cities
WHERE longitude < (
    SELECT longitude
    FROM north_american_cities
    WHERE city = 'Chicago'
)
ORDER BY longitude;

And, voilà:

city	country	population	latitude	longitude
Los Angeles	United States	3884307	34.052234	-118.243685
Phoenix	United States	1513367	33.448377	-112.074037
Guadalajara	Mexico	1500800	20.659699	-103.349609
Mexico City	Mexico	8555500	19.432608	-99.133208
Ecatepec de Morelos	Mexico	1742000	19.601841	-99.050674
Houston	United States	2195914	29.760427	-95.369803

The example is taken from sqlbolt.com.