SELECT statement

The SELECT statement is the SQL DML command for retrieving rows from one or more tables. Nearly every interactive query is some variant of SELECT some_columns FROM some_table WHERE some_condition.

The simplest form pulls named columns from a single table:

SELECT first_name, last_name FROM employees;

This returns every row in employees, but only the two named columns. To get every column, use *:

SELECT * FROM employees;

A WHERE clause narrows down which rows are returned (see WHERE clause):

SELECT * FROM employees
WHERE first_name = 'Khalid' OR first_name = 'Maris';

String comparison case-sensitivity depends on the dialect and collation. MySQL’s default collations are case-insensitive (_ci) so 'khalid' = 'Khalid' is true. PostgreSQL is case-sensitive by default; use LOWER() or ILIKE for case-insensitive matching. SQLite is case-sensitive for = on text but case-insensitive for ASCII characters under LIKE. SQL Server depends on the database’s collation setting. Always check before relying on either behaviour.

Patterns inside strings are matched with the LIKE operator:

SELECT * FROM employees WHERE first_name LIKE 'Mo%';

% matches any sequence of characters; _ matches exactly one character. So %mo matches names ending in mo, mo% matches names starting with mo, and %mo% matches names containing mo anywhere.

Beyond WHERE, real queries use other clauses:

• JOIN combines multiple tables on matching columns. The combining logic is what makes a relational database worth using.
• GROUP BY aggregates rows that share a value in some column, so we can compute counts, sums, averages per group.
• HAVING filters the aggregated groups, the way WHERE filters individual rows.
• ORDER BY sorts the result.
• LIMIT returns only the first $N$ rows.

Interactive querying is iterative: start with a broad SELECT *, narrow with WHERE, project the columns we actually need, aggregate or join as the question demands. SQL is a declarative language. We describe what rows we want, not how to find them, and the database engine plans the actual access strategy.