A query requests specific data from a database. In SQL, a query uses SELECT to specify columns, FROM to specify tables, and WHERE to filter rows. JOIN combines tables. GROUP BY aggregates. ORDER BY sorts. The database parses the query, plans an execution strategy, and returns the results. The same question can be asked in many ways, and some ways are much faster than others.

Query performance depends on several factors. Indexes speed up lookups on filtered columns. The query planner chooses between different execution strategies based on statistics about the data. Outdated statistics lead to poor plans. Writing a query that uses indexes effectively is a skill. A function applied to an indexed column in a WHERE clause prevents the index from being used. A query that selects all columns when only a few are needed wastes bandwidth and memory. A subquery that runs once per row is slow. The same logic as a join is fast. Query optimization is part art, part science. The database provides tools: EXPLAIN plans, query profilers, and performance counters. Reading those tools takes practice. A query that runs in milliseconds on a small dataset can take minutes on a large one. The difference is often the plan, not the hardware.

Query components

• SELECT — columns to return
• FROM — tables to query
• WHERE — row filters
• JOIN — combine tables
• GROUP BY — aggregate rows
• ORDER BY — sort results

A query is a question. The database answers it. The quality of the answer depends on how the question is asked.