MySQL 1Z0-909 Glossary: Development and Performance Terms

Use this glossary to clean up high-confusion SQL and schema terms, then route misses back to the right support page.

High-value terms

• Cardinality: The estimated or actual number of rows flowing through part of a query plan.
• Constraint: A schema rule such as primary key, unique, foreign key, or not null.
• EXPLAIN: The tool that shows how MySQL expects to execute a query.
• Isolation level: The rule set controlling which transaction changes can be seen by other transactions.
• JSON path: The expression syntax used to locate values inside JSON documents.
• Sargable predicate: A condition that the optimizer can use efficiently with an index.
• Stored program: A database-side object such as a procedure, function, trigger, or event.
• Transaction: A unit of work bounded by commit or rollback.
• Window function: A function that computes across a logical window of rows without collapsing the result set the way aggregation does.
• Write amplification: Extra work created by schema or indexing choices that make writes more expensive.

Common confusion pairs

• WHERE vs HAVING: WHERE filters rows before grouping. HAVING filters groups after aggregation.
• Index use vs full scan: A matching index path is often safer, but only if the predicate shape stays sargable.
• Join bug vs schema bug: Some wrong answers come from bad join logic. Others come from missing keys or wrong data shape.
• Stored procedure vs trigger: A procedure is called explicitly. A trigger runs automatically on table events.
• Transaction isolation vs locking: Isolation describes visibility rules. Locking is one mechanism that affects behavior under concurrency.

Where to review next

• Weekly sequence and weak-spot planning: Study Plan
• SQL, optimizer, and transaction traps: Cheat Sheet
• Last-week questions: FAQ
• Canonical Oracle and MySQL references: Resources