SQL Tutorial

The Language of Databases SQL (Structured Query Language) is the standard language for creating, reading, updating and deleting data in relational databases. Almost every modern ...

Reading Data SELECT is the most used SQL statement. It asks the database for rows and columns. Aliases Make Output Readable The AS keyword renames a column in the result set. Some ...

Narrowing the Result Set WHERE filters rows before you see them. Only rows for which the condition is true come back. Comparison Operators = , <> or != for equality/inequality. > ...

Turning Many Rows Into One Summary Aggregate functions collapse a column of values into a single number. The Core Five COUNT(*): number of rows; COUNT(col) ignores NULLs. SUM: ...

Splitting Rows Into Groups GROUP BY divides rows into groups, then an aggregate runs once per group - one output row per group. The Golden Rule Every column in the SELECT list ...

Why We Join Good databases keep data in separate tables. Joins re-combine those tables so you can answer questions that span them, like "which customers bought which products". ...

A Query Inside a Query A subquery is a SELECT nested inside another statement. It computes a value, a list or a table that the outer query uses. Subquery Returning One Value ...

Combining Whole Result Sets Set operators glue together two result sets that have the same columns. UNION and UNION ALL UNION removes duplicate rows; UNION ALL keeps everything ...

Adding Data With INSERT Updating Data Never run UPDATE without a WHERE clause unless you really mean every row. Deleting Data Copying Between Tables Managing Conflicts (Upsert) ...

Changing the Structure, Not the Data DDL (Data Definition Language) manages tables and other schema objects. CREATE TABLE ALTER TABLE The exact ALTER syntax varies by database; ...

Rules Written Into the Database Constraints enforce integrity at the database level so invalid data can never be stored, no matter which application sends it. The Constraint ...

A Saved Query You Can Query A view is a SELECT with a name. It behaves like a table for SELECT purposes but does not store its own data. Why Views Are Useful Simplicity: hide ...

Work in Safe Batches A transaction bundles several statements into one all-or-nothing unit. If anything fails, the whole batch rolls back. This is why transferring money between ...

Calculations Across Neighbouring Rows Window functions compute a value over a window of rows while keeping every row visible - unlike GROUP BY, which collapses them. Ranking Rows ...

Why Queries Get Slow Without an index, the database scans every row to find matches (a full table scan). An index is an ordered structure that lets it jump straight to the ...

Think Before You CREATE A good schema makes data honest and queries easy. A poor one produces duplicates, contradictions and painfully slow reports. Normalization Basics 1NF: ...