PostgreSQL Select Data

PostgreSQL Tutorial

🔍 PostgreSQL Select Data (Beginner → Advanced)

In PostgreSQL Select Data is used to retrieve (read) data from one or more tables.
It is the most frequently used SQL command and the foundation of database querying.

1️⃣ What is SELECT in PostgreSQL?

SELECT is used to fetch data from database tables.

SELECT column_name FROM table_name;

2️⃣ Select All Data ⭐

SELECT * FROM students;

✔ Fetches all columns and all rows
⚠️ Avoid in large tables (performance issue)

3️⃣ Select Specific Columns ⭐ (Recommended)

SELECT name, marks FROM students;

✔ Faster
✔ Cleaner output
✔ Best practice

4️⃣ SELECT with WHERE (Filtering Data) 🔍


Common Operators

Operator Meaning
= Equal
!= / <> Not equal
> < Greater / Less
>= <= Greater / Less equal
BETWEEN Range
IN Multiple values
LIKE Pattern matching

5️⃣ SELECT with AND, OR, NOT ⭐


6️⃣ SELECT DISTINCT (Remove Duplicates) ⭐

SELECT DISTINCT class FROM students;

✔ Returns unique values only

7️⃣ SELECT with ORDER BY 📊


✔ Default is ASC

8️⃣ SELECT with LIMIT and OFFSET ⭐


✔ Used for pagination

9️⃣ SELECT with Aggregate Functions 🔢

Function Purpose
COUNT() Count rows
SUM() Total
AVG() Average
MIN() Minimum
MAX() Maximum

🔟 SELECT with GROUP BY ⭐


✔ Groups rows by column
✔ Used with aggregate functions

1️⃣1️⃣ HAVING Clause (Filter Groups) ⚠️


📌 WHERE → filters rows
📌 HAVING → filters groups

1️⃣2️⃣ Column & Table Aliases (AS) ⭐


✔ Improves readability
✔ AS keyword is optional

1️⃣3️⃣ SELECT with LIKE (Pattern Matching)


Pattern Meaning
% Any number of characters
_ Exactly one character

1️⃣4️⃣ SELECT with IN and BETWEEN


1️⃣5️⃣ SELECT with JOIN 🔗 (Very Important)


✔ Retrieves data from multiple tables

🔥 Order of Execution in PostgreSQL SELECT

FROM
WHERE
GROUP BY
HAVING
SELECT
ORDER BY
LIMIT / OFFSET

✔ Frequently asked in interviews

❌ Common Mistakes

❌ Using WHERE after GROUP BY
❌ Forgetting ORDER BY before LIMIT
❌ Using = instead of LIKE
❌ Misusing HAVING
❌ Using SELECT * everywhere

📌 Interview Questions (Must Prepare)

  1. How does SELECT work in PostgreSQL?

  2. Difference between WHERE and HAVING

  3. SELECT DISTINCT vs GROUP BY

  4. Execution order of SELECT

  5. How pagination works using LIMIT and OFFSET

  6. How to fetch data from multiple tables?

✅ Summary

✔ SELECT is used to read data
✔ Supports filtering, sorting, grouping
✔ PostgreSQL supports LIMIT & OFFSET
✔ Aggregates + GROUP BY are powerful
✔ Joins allow multi-table queries
✔ Core skill for PostgreSQL, backend & interviews

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