T-SQL: Essential Tips for Beginners

Key insights

• T-SQL overview: T-SQL is Microsoft’s SQL extension used in SQL Server and Azure SQL.
  Focus on preventing silent mistakes that return wrong results for data analysis and Power BI reports.
• Use AS with column aliases: Always use AS (or explicit alias syntax) so column names and expressions stay clear.
  Missing commas or unclear aliases can make columns return unexpected values or hide errors.
• COUNT(*) vs COUNT(column): Use COUNT(*) to count all rows and COUNT(column) to count only non-NULL values.
  Pick the right form to avoid under- or over-counting in reports and aggregations.
• LEFT JOIN and filtering pitfalls: Applying a WHERE condition on the right-side table after a LEFT JOIN can remove rows and act like an inner join.
  Move filters into the JOIN condition or explicitly handle NULLs to preserve left-side rows.
• Grouping and aggregates: Group dates at the correct level (year, month, day) to avoid wrong totals or missed rows.
  Use HAVING to filter aggregated results and GROUP BY for correct aggregation, not WHERE.
• Practical best practices: Avoid SELECT *, use CTEs to simplify complex queries, and encapsulate logic in stored procedures when needed.
  Use TRY...CATCH, transactions, consistent formatting, and practice in tools like SSMS or Azure Data Studio to build reliable skills.

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