MySQL Fixes

9 troubleshooting guides — each with the error, why it happens, and copy-paste commands to fix it.

• MySQL: "Server Has Gone Away" (Error 2006) — The Four Real Causes — The server closed the connection (or it never survived).
• MySQL: "Lock Wait Timeout Exceeded; Try Restarting Transaction" — Your transaction waited innodb_lock_wait_timeout (default 50s) for a row another transaction holds.
• MySQL "Access denied for user" — Grant, Host, and Password Traps — Access denied means MySQL matched you to a user@host account and rejected the credentials — or no such account exists for your host.
• MySQL "Error 1040: Too many connections" — max_connections (often 151 by default!) is exhausted.
• MySQL "InnoDB: Unable to lock ./ibdata1" — A Second Server Is Running — Only one mysqld can own the data files.
• MySQL "Can't Connect to Local Server Through Socket" — Where the Socket Went — The client looked for a unix socket file that isn't there — mysqld is down, or it uses a different socket path than the client expects.
• MySQL "Unknown Database" — Names, Grants, and Case Sensitivity — The server doesn't know the database you named: it doesn't exist, or the lowercase/UTF8 quirks of identifiers bit you.
• MySQL "Deadlock Found When Trying to Get Lock" — Reading thevictim Trace — Two transactions each hold locks the other needs; InnoDB kills one (the victim).
• MySQL "Duplicate Entry for Key PRIMARY" (Error 1062) — A unique key collided.