十一個高級MySql筆試題
1. Explain MySQL architecture. – The front layer takes care of network connections and security authentications, the middle layer does the SQL query parsing, and then the query is handled off to the storage engine. A storage engine could be either a default one supplied with MySQL (MyISAM) or a commercial one supplied by a third-party vendor (ScaleDB, InnoDB, etc.)
2. Explain MySQL locks. – Table-level locks allow the user to lock the entire table, page-level locks allow locking of certain portions of the tables (those portions are referred to as tables), row-level locks are the most granular and allow locking of specific rows.
3. Explain multi-version concurrency control in MySQL. – Each row has two additional columns associated with it – creation time and deletion time, but instead of storing timestamps, MySQL stores version numbers.
4. What are MySQL transactions? – A set of instructions/queries that should be executed or rolled back as a single atomic unit.
5. What’s ACID? – Automicity – transactions are atomic and should be treated as one in case of rollback. Consistency – the database should be in consistent state between multiple states in transaction. Isolation – no other queries can access the data modified by a running transaction. Durability – system crashes should not lose the data.
6. Which storage engines support transactions in MySQL? – Berkeley DB and InnoDB.
7. How do you convert to a different table type? – ALTER TABLE customers TYPE = InnoDB
8. How do you index just the first four bytes of the column? – ALTER TABLE customers ADD INDEX (business_name(4))
9. What’s the difference between PRIMARY KEY and UNIQUE in MyISAM? – PRIMARY KEY cannot be null, so essentially PRIMARY KEY is equivalent to UNIQUE NOT NULL.
10. How do you prevent MySQL from caching a query? – SELECT SQL_NO_CACHE …
11. What’s the difference between query_cache_type 1 and 2? – The second one is on-demand and can be retrieved via SELECT SQL_CACHE … If you’re worried about the SQL portability to other servers, you can use SELECT /* SQL_CACHE */ id FROM … – MySQL will interpret the code inside comments, while other servers will ignore it.
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