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MySQL Tips and Tricks

mysql2 min read

Last updated : 06/July/2020

Events are in reverse chronological order.

Lock wait timeout exceeded

Got the above error message, when executing an UPDATE statement in MySQL terminal. Upon a bit of analysis found that, a program was abnormally terminated and it had the thread still active, could see in SHOW PROCESSLIST;. So after killing the thread, UPDATE statement was reexecuted and completed successfully in few seconds.

Creating test user with full access on test database

After creating the user, can be tested by opening a new command prompt,

For python, you may need to use mysql_native_password, i have written a separate blog post for this.

MySQL commonly used commands

Virtual columns

They appear to be normal columns but their values are calculated or derived from other columns and they are not materialized insense they are not stored in disk.

In my usecase, i find them extremely useful when dealing with JSON columns as JSON queries are tend to get lot bigger and complex. So, this virtal column greatly helps. Heres a sample,

Here is a simple JSON query retreving usb > -1

Lets add virtual column, to simplify the query,

Now all we have to do is

Viewing specfic range of output by using OFFSET

Using offset we can review a big result or table part by part in the terminal which doesn't have scrollbars

What below query does is,

  • LIMIT 5 : Limits number of rows to be retreived to 5
  • OFFSET 5: Retreive rows after skipping 5 rows from the resultset

Output in vertical format

Usually when we are retreiving data from table we expect to be in table format but at certain times, when we want to report or note it down, it will be better if its in 'vertial format'.

Default format

Vertical format

on duplicate update

VALUES(col_name) in the ON DUPLICATE KEY UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred

Above when it says, "2 rows affected", its not two rows, its just value 2, for example,

  • if 0, had come, it means nothing is udpated
  • 1 means row is inserted
  • 2 means row is updated.

Its confusing a bit here but it is how it is.

Row change timestamp

Represents timestamp, when the row was last changed.

Running MySQL script from windows prompt

Below is the load card to truncate and load data into mySQL table

Below windows batch file to execute the above load control card,

FYI : Load card - Its a mainframe term meaning a member(equivalent to file in folder) containing load statement.

Loading mysql tables

Line number and description,

  1. Input file path, its Windows, so remember its a forward slash
  2. IGNORE ignores duplicates while loading into table
  3. Columns are terminated by pipe symbol(|) rather than comma(,) as unitName can contain commas.
  4. Ignore the header line
  5. Table Column names as they appear in the delimited file

Updating values while loading the table

SET clause can be used for this purpose, all MySQL functions can be used.