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.

 What is SELECT in PostgreSQL?

SELECT is used to fetch data from database tables.

Select All Data

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

 Select Specific Columns (Recommended)

  •  Faster
  • Cleaner output
  • Best practice

SELECT with WHERE (Filtering Data)

Common Operators

OperatorMeaning
=Equal
!= / <>Not equal
> <Greater / Less
>= <=Greater / Less equal
BETWEENRange
INMultiple values
LIKEPattern matching

SELECT with AND, OR, NOT

SELECT DISTINCT (Remove Duplicates)

  •  Returns unique values only

SELECT with ORDER BY

  •  Default is ASC

SELECT with LIMIT and OFFSET

  • Used for pagination

SELECT with Aggregate Functions

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

SELECT with GROUP BY

  •  Groups rows by column
  •  Used with aggregate functions

HAVING Clause (Filter Groups)

  • WHERE → filters rows
  • HAVING → filters groups

 Column & Table Aliases (AS)

  •  Improves readability
  • AS keyword is optional

Using LIKE for Pattern Matching

PatternMeaning
%Any number of characters
_Exactly one character

Filtering Data with IN and BETWEEN Clauses

Understanding JOIN in SQL (Very Important Concept)

  • 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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