Lecture 02: Being Selective with SQL

DATA 351: Data Management with SQL

Lucas P. Cordova, Ph.D.

lpcordova@willamette.edu

Willamette University

January 14, 2026

Lecture Outline

• Announcements
  • Review
  • Selecting Columns
  • Filtering with WHERE
  • Combining Conditions
  • Sorting with ORDER BY
  • Limiting Results
  • Putting It All Together
  • Practice Problems
  • Common Mistakes
  • Summary

Announcements

Today’s Agenda

• We will cover all the SQL you need for the assignment today

• Topics:

  • SELECT and column aliases
  • WHERE clause and filtering
  • Combining conditions with AND/OR
  • ORDER BY for sorting
  • LIMIT for restricting results

About Homework 1

• Uses the Stack Overflow 2025 Developer Survey dataset
• 10 SQL queries exploring developer demographics, AI attitudes, and work preferences
• Autograded through CodeGrade
• Each query builds on today’s concepts

Review

Quick Review

What do you think this query will return?

SELECT * FROM survey25;

All columns and all rows from the survey25 table.

Review Question

Which SQL keyword do you think you would use to retrieve only the unique values in a column?

A. UNIQUE

B. SINGLE

C. DISTINCT

D. DIFFERENT

Answer: C - DISTINCT returns only unique values

Selecting Columns

The SELECT Statement

The most fundamental SQL operation is selecting data from a table.

Basic syntax:

SELECT column1, column2, column3
FROM table_name;

This returns the specified columns for every row in the table.

Selecting All Columns

Use the asterisk * to select all columns:

SELECT *
FROM survey25;

Useful for exploration, but in practice you should specify only the columns you need.

Selecting Specific Columns

For the developer survey, we might want just a few columns:

SELECT response_id, age, country
FROM survey25;

This returns only those three columns for all respondents.

Why Be Selective?

Selecting only needed columns has benefits:

• Performance: Less data to transfer and process
• Clarity: Results are easier to read and understand
• Memory: Reduces memory usage for large tables

Column Aliases with AS

Sometimes column names are unclear or too long. Use AS to rename them in your output:

SELECT response_id AS id,
       age AS age_range,
       country AS location
FROM survey25;

The original table is unchanged; only the output uses the new names.

Alias Syntax Options

Both of these work in PostgreSQL:

-- With AS keyword (preferred for clarity)
SELECT response_id AS id FROM survey25;

-- Without AS keyword (also valid)
SELECT response_id id FROM survey25;

Use AS for readability.

When to Use Aliases

Aliases are helpful when:

• Column names are cryptic (i_cor_pm becomes role_type)
• You want more descriptive output (converted_comp_yearly becomes salary_usd)
• Column names contain special characters
• You need to distinguish columns from different tables (joins)

Think-Pair-Share: Column Selection

Think (1 min): Write a query to select the dev_type and remote_work columns from survey25, renaming them to job_title and work_style.

Pair (2 min): Compare your answer with a neighbor.

Share: Let’s see some solutions!

SELECT dev_type AS job_title,
       remote_work AS work_style
FROM survey25;

Filtering with WHERE

Why Filter?

Real databases have thousands or millions of rows. You rarely want all of them.

The WHERE clause filters rows based on conditions:

SELECT column1, column2
FROM table_name
WHERE condition;

Basic Equality Filter

Find all remote workers:

SELECT response_id, dev_type, country
FROM survey25
WHERE remote_work = 'Remote';

Note: String values must be in single quotes.

Comparison Operators

SQL provides several comparison operators:

Operator	Meaning
=	Equal to
<> or !=	Not equal to
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal

Numeric Comparisons

Find developers with more than 15 years of work experience:

SELECT response_id, work_exp, dev_type
FROM survey25
WHERE work_exp > 15;

No quotes needed for numbers.

String Comparisons

Find developers in Canada:

SELECT response_id, country, dev_type
FROM survey25
WHERE country = 'Canada';

String comparisons are case-sensitive by default.

Understanding Check

What will this query return?

SELECT response_id, age
FROM survey25
WHERE age = '18-24 years old';

A. All developers aged exactly 18 to 24

B. All developers with age column containing that exact string

C. An error because age should be a number

D. Nothing because no one is exactly that age

Answer: B - The age column contains text values like ‘18-24 years old’

The Problem with NULL

NULL represents missing or unknown data. It behaves strangely:

-- This will NOT work as expected!
SELECT * FROM survey25
WHERE converted_comp_yearly = NULL;

This returns zero rows because NULL comparisons always return NULL, not true or false.

IS NULL and IS NOT NULL

To check for NULL values, use special syntax:

-- Find rows WITH salary data
SELECT response_id, converted_comp_yearly
FROM survey25
WHERE converted_comp_yearly IS NOT NULL;

-- Find rows WITHOUT salary data
SELECT response_id, converted_comp_yearly
FROM survey25
WHERE converted_comp_yearly IS NULL;

NULL Visualization

Think-Pair-Share: Filtering

Think (1 min): Write a query to find all developers who believe AI is a threat to their job (ai_threat = 'Yes').

Pair (2 min): Share with a neighbor.

Share: What columns would be interesting to include?

SELECT response_id, age, dev_type, ai_threat
FROM survey25
WHERE ai_threat = 'Yes';

Combining Conditions

Multiple Conditions with AND

Often you need to filter on multiple criteria. Use AND when all conditions must be true:

SELECT response_id, age, main_branch
FROM survey25
WHERE age = '18-24 years old'
  AND main_branch = 'I am learning to code';

AND Logic Visualized

Multiple Conditions with OR

Use OR when any condition being true is sufficient:

SELECT response_id, remote_work
FROM survey25
WHERE remote_work = 'Remote'
   OR remote_work = 'Hybrid';

AND vs OR

Operator	Behavior	Use When
AND	All conditions must be true	Narrowing results
OR	At least one must be true	Expanding results

Order of Operations

AND has higher precedence than OR. Use parentheses to be explicit:

-- Without parentheses (AND evaluated first)
SELECT * FROM survey25
WHERE industry = 'Software Development'
  AND ai_threat = 'Yes'
   OR work_exp > 20;

-- With parentheses (clearer intent)
SELECT * FROM survey25
WHERE industry = 'Software Development'
  AND (ai_threat = 'Yes' OR work_exp > 20);

Real Example: AI Enthusiasts

Find developers who use AI daily AND have very favorable AI sentiment:

SELECT response_id, dev_type, years_code AS coding_experience
FROM survey25
WHERE ai_select = 'Yes, I use AI tools daily'
  AND ai_sent = 'Very favorable';

Understanding Check

How many conditions must be true for a row to be returned?

SELECT * FROM survey25
WHERE country = 'United States'
  AND work_exp > 10
  AND remote_work = 'Remote';

A. At least one

B. At least two

C. All three

D. Exactly one

Answer: C - With AND, all conditions must be true

Think-Pair-Share: Combined Filters

Think (2 min): Write a query to find full-stack developers (dev_type = 'Developer, full-stack') with more than 15 years of experience.

Pair (2 min): Compare queries with your neighbor.

SELECT response_id, work_exp, org_size
FROM survey25
WHERE dev_type = 'Developer, full-stack'
  AND work_exp > 15;

Sorting with ORDER BY

Why Sort Results?

Without explicit ordering, SQL returns rows in an unpredictable order.

Use ORDER BY to sort your results:

SELECT column1, column2
FROM table_name
ORDER BY column1;

Ascending Order (Default)

Sort developers by years of experience (lowest first):

SELECT response_id, work_exp, dev_type
FROM survey25
WHERE work_exp IS NOT NULL
ORDER BY work_exp;

ASC (ascending) is the default and can be omitted.

Descending Order

Sort by salary (highest first):

SELECT response_id, country, converted_comp_yearly AS salary_usd
FROM survey25
WHERE converted_comp_yearly IS NOT NULL
ORDER BY converted_comp_yearly DESC;

ORDER BY with Aliases

You can order by a column alias:

SELECT response_id, 
       converted_comp_yearly AS salary_usd
FROM survey25
WHERE converted_comp_yearly IS NOT NULL
ORDER BY salary_usd DESC;

Or use the original column name. Both work.

Sorting Text Columns

Text is sorted alphabetically:

SELECT DISTINCT country
FROM survey25
ORDER BY country;

Sorting depends on the database collation (locale settings).

NULL Values in Sorting

By default in PostgreSQL:

• ASC: NULLs appear last
• DESC: NULLs appear first

You can control this:

ORDER BY column ASC NULLS FIRST
ORDER BY column DESC NULLS LAST

Sorting Flow

ORDER BY is processed after filtering.

Limiting Results

The LIMIT Clause

Sometimes you only want a few rows. Use LIMIT:

SELECT response_id, country, converted_comp_yearly AS salary_usd
FROM survey25
WHERE converted_comp_yearly IS NOT NULL
ORDER BY converted_comp_yearly DESC
LIMIT 10;

This returns only the top 10 highest earners.

LIMIT Position

LIMIT always comes last in the query:

SELECT columns
FROM table
WHERE conditions
ORDER BY column
LIMIT n;

Common LIMIT Use Cases

LIMIT is useful for:

• Finding top N or bottom N results
• Sampling data to understand table structure
• Pagination (showing results in pages)
• Testing queries on large tables

Top 5 Happiest Developers

SELECT response_id, 
       job_sat AS satisfaction_score,
       dev_type,
       remote_work AS work_arrangement
FROM survey25
WHERE job_sat IS NOT NULL
ORDER BY job_sat DESC
LIMIT 5;

Understanding Check

What does this query return?

SELECT response_id, work_exp
FROM survey25
ORDER BY work_exp ASC
LIMIT 3;

A. The 3 most experienced developers

B. The 3 least experienced developers

C. 3 random developers

D. An error

Answer: B - ASC sorts lowest first, LIMIT 3 takes the first three

Putting It All Together

Query Order of Execution

When SQL runs your query, it processes clauses in this order:

1. FROM - Which table?
2. WHERE - Which rows match?
3. SELECT - Which columns?
4. ORDER BY - How to sort?
5. LIMIT - How many rows?

Query Writing Order

You write queries in this order:

SELECT columns      -- 1st: What to show
FROM table          -- 2nd: Where from
WHERE conditions    -- 3rd: Which rows
ORDER BY column     -- 4th: How to sort
LIMIT n;            -- 5th: How many

Complete Example

Find the top 10 highest-paid developers with valid salary data:

SELECT response_id,
       country,
       dev_type,
       converted_comp_yearly AS salary_usd
FROM survey25
WHERE converted_comp_yearly IS NOT NULL
ORDER BY converted_comp_yearly DESC
LIMIT 10;

Query Building Strategy

Think-Pair-Share: Complete Query

Think (3 min): Write a query to find developers with less than 3 years of experience at companies with 10,000+ employees. Order by experience (lowest first).

Pair (2 min): Compare with your neighbor.

SELECT response_id, age, ed_level, work_exp, org_size
FROM survey25
WHERE work_exp < 3
  AND org_size = '10,000 or more employees'
ORDER BY work_exp ASC;

Practice Problems

Practice 1: Stack Overflow Power Users

Write a query to find developers who:

• Visit Stack Overflow multiple times per day
• Definitely consider themselves community members

Show their response_id, years_code, and relevant columns with good aliases.

SELECT response_id,
       years_code,
       so_visit_freq AS visit_frequency,
       so_comm AS community_member
FROM survey25
WHERE so_visit_freq = 'Multiple times per day'
  AND so_comm = 'Yes, definitely';

Practice 2: AI Skeptics

Write a query to find developers in Software Development who believe AI threatens their job.

SELECT response_id,
       age,
       dev_type,
       ai_threat AS ai_job_threat
FROM survey25
WHERE industry = 'Software Development'
  AND ai_threat = 'Yes';

Practice 3: Happy Remote Workers

Write a query to find the 5 happiest developers. Exclude NULL satisfaction scores.

SELECT response_id,
       job_sat AS satisfaction_score,
       dev_type,
       remote_work AS work_arrangement
FROM survey25
WHERE job_sat IS NOT NULL
ORDER BY job_sat DESC
LIMIT 5;

Common Mistakes

Mistake 1: Wrong Quotes

-- WRONG: Double quotes for strings
SELECT * FROM survey25 WHERE country = "Canada";

-- CORRECT: Single quotes for strings
SELECT * FROM survey25 WHERE country = 'Canada';

Double quotes are for identifiers (column/table names), not string values.

Mistake 2: NULL Comparisons

-- WRONG: Using = with NULL
SELECT * FROM survey25 WHERE job_sat = NULL;

-- CORRECT: Using IS NULL
SELECT * FROM survey25 WHERE job_sat IS NULL;

-- CORRECT: Using IS NOT NULL
SELECT * FROM survey25 WHERE job_sat IS NOT NULL;

Mistake 3: Case Sensitivity

-- May return nothing if data is 'Remote'
SELECT * FROM survey25 WHERE remote_work = 'remote';

-- Exact match required
SELECT * FROM survey25 WHERE remote_work = 'Remote';

String comparisons are case-sensitive. Check your data values.

Mistake 4: Missing Quotes

-- WRONG: String without quotes
SELECT * FROM survey25 WHERE country = Canada;

-- CORRECT: String with quotes
SELECT * FROM survey25 WHERE country = 'Canada';

-- CORRECT: Numbers don't need quotes
SELECT * FROM survey25 WHERE work_exp > 10;

Mistake 5: Wrong Clause Order

-- WRONG: LIMIT before ORDER BY
SELECT * FROM survey25
LIMIT 10
ORDER BY work_exp;

-- CORRECT: ORDER BY before LIMIT
SELECT * FROM survey25
ORDER BY work_exp
LIMIT 10;

Summary

Key Concepts

Today we covered:

• SELECT specifies which columns to return
• AS creates column aliases for clearer output
• WHERE filters rows based on conditions
• AND/OR combine multiple conditions
• IS NULL and IS NOT NULL handle missing data
• ORDER BY sorts results (ASC or DESC)
• LIMIT restricts the number of rows returned

Query Template

SELECT column1, 
       column2 AS alias
FROM table_name
WHERE condition1
  AND condition2
ORDER BY column1 DESC
LIMIT n;

For Homework 1

You now have all the tools needed:

• 10 queries using the Stack Overflow survey
• Each builds on these concepts
• Test your queries before submitting
• Pay attention to exact column aliases
• String values must match exactly

Next Class

We will cover:

• More advanced filtering with BETWEEN and IN
• Pattern matching with LIKE and ILIKE
• Working with dates and times

Read Chapter 3 before next class.