Sql Query To Obtain Values That Occur More Than Once

Break down the basics: Demystifying Duplication

Core SQL elements: Unpacking GROUP BY and COUNT

Leverage the GROUP BY clause to accumulate rows with similar values into aggregate rows. Supplement this operation with COUNT() to tally the frequency of each value in your column.

Opt for efficiency: Prioritize relevant columns

Trim the fat with SELECT specific_columns, avoiding SELECT * to bump up performance by narrowing down data processed and transmitted.

Fine tune with subqueries

Intensify your query with the subtle art of subqueries. Let's focus on LastName that appear thrice or more:

/* You shout 'em out */
SELECT LastName, COUNT(LastName) AS CountOf
FROM Employees
GROUP BY LastName
HAVING COUNT(LastName) > 2;


Upgrade your strategy: Advanced SQL Techniques

Enhance readability with CTEs

Venture into realm of Common Table Expressions (CTEs) to arrange complex queries cleanly:

WITH RepeatedValuesCTE (Value, Occurrence) AS (
  SELECT Value, COUNT(*) /* Let's count 'em */
  FROM Table
  GROUP BY Value
  HAVING COUNT(*) > 1 /* Time to pick 'em out */
)

/* Now watch your data groove */
SELECT *
FROM RepeatedValuesCTE;


CTEs serve to define a temporary result set making your queries lean and clean.

Dabble with window functions

Pair COUNT() with OVER and PARTITION BY to compute detailed group tallies:

SELECT LastName,
       COUNT(*) OVER (PARTITION BY LastName) AS CountOf /* they are not alone */
FROM Employees
WHERE COUNT(*) OVER (PARTITION BY LastName) > 2; /* Three's not a crowd here */


Boost clarity with aliases

Bestow aliases to your columns, more so in subqueries, to refine readability and rule out any confusion.

Leverage ORDER BY

Incorporate ORDER BY with COUNT(*) to exhibit results based on frequency:

SELECT column, COUNT(*) AS CountOf
FROM table
GROUP BY column
HAVING COUNT(*) > 1
ORDER BY CountOf DESC;


Play it smooth with the IN operator

Need to filter results based on a list from a subquery? Use the IN operator:

/* let's call them the IN crowd */
SELECT *
FROM table
WHERE column IN (
  SELECT column
  FROM table
  GROUP BY column
  HAVING COUNT(*) > 1
);


Dodging lurking nested query performance issues has never been easier.