CAST and CONVERT Examples in SQL

What CAST and CONVERT does

Change a value from one data type to another. SQL syntax can vary by database, but the pattern below is a useful starting point for reports and analysis.

Syntax or pattern

CAST(column_name AS data_type)

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5 practical examples

Use CAST and CONVERT in a sales report

Apply the CAST and CONVERT pattern to a sales table.

-- CAST and CONVERT example for sales
SELECT customer_id, order_date, total_amount
FROM orders
WHERE total_amount > 100;

This shows how the CAST and CONVERT pattern can support a simple sales analysis.

Use CAST and CONVERT for customers

Apply the CAST and CONVERT pattern to customer records.

-- CAST and CONVERT example for customers
SELECT customer_id, email, status
FROM customers
WHERE status = 'Active';

This is useful when customer records need filtering, labeling or summarizing.

Use CAST and CONVERT for products

Apply the CAST and CONVERT pattern to product or inventory data.

-- CAST and CONVERT example for products
SELECT product_id, product_name, category
FROM products;

Product tables are good practice data for this SQL pattern.

Use CAST and CONVERT for monthly reporting

Apply the CAST and CONVERT pattern to a monthly reporting query.

-- CAST and CONVERT example for monthly reporting
SELECT DATE_TRUNC('month', order_date) AS month, SUM(total_amount) AS sales
FROM orders
GROUP BY DATE_TRUNC('month', order_date);

This turns row-level transactions into a report-friendly result.

Use CAST and CONVERT during data checks

Apply the CAST and CONVERT pattern to find data quality issues.

-- CAST and CONVERT example for data checks
SELECT customer_id, COUNT(*) AS records
FROM orders
GROUP BY customer_id
HAVING COUNT(*) > 1;

This is a useful pattern for auditing data before building a report.

Common mistakes to avoid

Forgetting that SQL dialects vary across PostgreSQL, SQL Server, MySQL, BigQuery and SQLite.
Using SELECT * in production reports when only a few columns are needed.
Not checking join keys, duplicate rows or NULL values before trusting results.

FAQ

Will this SQL work in every database?

The idea is portable, but function names and date syntax may vary. Check your database dialect if a function is not recognized.

Should I use this in a report query?

Yes, if the pattern matches the business question and you have checked filters, joins and row counts.

Why does my result have too many rows?

The most common reasons are duplicate join keys, missing filters or grouping at the wrong level of detail.

💡 Useful resources

Here are some ideas for you

Optional resources that may help if you are learning SQL, building reports, writing queries or improving your data workflow.

📘
SQL books for beginners
Practice query patterns with structured examples and exercises.
See ideas
🧱
Database design books
Understand tables, keys, relationships and why joins behave the way they do.
See ideas
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Mechanical keyboards
Useful if you write queries, code and documentation for long work sessions.
See ideas
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External monitors
View query editor, result grid and documentation side by side.
See ideas
📒
Developer notebooks
Sketch table relationships, query logic and report ideas before coding.
See ideas
💡
Desk lamps
Keep your workspace comfortable while studying or debugging queries.
See ideas

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