SQL Lesson 2: Queries with constraints (Pt. 1)

Now we know how to select for specific columns of data from a table, but if you had a table with a hundred million rows of data, reading through all the rows would be inefficient and perhaps even impossible.

In order to filter certain results from being returned, we need to use a WHERE clause in the query. The clause is applied to each row of data by checking specific column values to determine whether it should be included in the results or not.

Select query with constraints

SELECT column, another_column, …
FROM mytable
WHERE condition
    AND/OR another_condition
    AND/OR …;

More complex clauses can be constructed by joining numerous AND or OR logical keywords (ie. num_wheels >= 4 AND doors <= 2). And below are some useful operators that you can use for numerical data (ie. integer or floating point):

Operator	Condition	SQL Example
=, !=, < <=, >, >=	Standard numerical operators	col_name != 4
BETWEEN … AND …	Number is within range of two values (inclusive)	col_name BETWEEN 1.5 AND 10.5
NOT BETWEEN … AND …	Number is not within range of two values (inclusive)	col_name NOT BETWEEN 1 AND 10
IN (…)	Number exists in a list	col_name IN (2, 4, 6)
NOT IN (…)	Number does not exist in a list	col_name NOT IN (1, 3, 5)

In addition to making the results more manageable to understand, writing clauses to constrain the set of rows returned also allows the query to run faster due to the reduction in unnecessary data being returned.

Did you know?

As you might have noticed by now, SQL doesn't require you to write the keywords all capitalized, but as a convention, it helps people distinguish SQL keywords from column and tables names, and makes the query easier to read.

Exercise

Using the right constraints, find the information we need from the Movies table for each task below.

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Table: movies

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Exercise 2 — Tasks

Find the movie with a row id of 6
Find the movies released in the years between 2000 and 2010
Find the movies not released in the years between 2000 and 2010
Find the first 5 Pixar movies and their release year

Stuck? Read this task's Solution.
Solve all tasks to continue to the next lesson.

Finish above Tasks