Mysql Data Types and Sizes for String/ Text

Mysql Data Types and Sizes for String/ Text

Category : How-to

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MySQL or MariaDB has several data types for handling text-based characters. There are several data types for handling smaller strings such as CHAR and VARCHAR data types. For larger text-based strings there are BLOB based data types such as TEXT.

It’s worth noting at this point that the below-quoted sizes do not necessarily represent the number of characters they can hold. In addition, more recent versions of MySQL (version 5 and 8) counts characters when defining the length, however, prior to these versions byres were used.

The below table shows the ‘size’ of each data type – notice that some data types are mentioned in characters, and others in bytes. The number of characters are always used when defining a string data type in your DDL statement – for example, VARCHAR(10).

Data TypeSizeDescription
CHAR(n)255 charactersFixed-length character field. Rows are padded with whitespace to the defined length.
VARCHAR(n)65,535 bytes *Variable-length character field with no manipulation on INSERT or SELECT.
TINYTEXT255 bytesVariable-length and stored off-row. Can only be sorted and grouped by up to max_sort_length characters
TEXT(n)65,535 bytes Variable-length and stored off-row. Can only be sorted and grouped by up to max_sort_length characters
MEDIUMTEXT(n)16,777,215 bytes (16MB)Variable-length and stored off-row. Can only be sorted and grouped by up to max_sort_length characters
LONGTEXT(n)4,294,967,295 bytes (4GB)Variable-length and stored off-row. Can only be sorted and grouped by up to max_sort_length characters
ENUM2 bytesThe ENUM doesn’t need a size definition but can hold up to 65,535 values.

* The maximum row length in MySQL is 65,535 bytes – your total row size cannot exceed this byte value. Keep in mind that some character sets consume more than one byte per character. For example, utf8mb4 can take up to 4 bytes per character and therefore the maximum VARCHAR is approximately one-quarter of the maximum row size.


Docker Compose yml for WordPress

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The below docker-compose yml file will create two Docker containers for running WordPress; a MySQL database and an Apache PHP web server.

version: '3.6'

services:
    db:
        image: mysql:5.7
        container_name: wp_mysql
        volumes:
          - ./data/mysql:/var/lib/mysql
        restart: unless-stopped
        environment:
            MYSQL_ROOT_PASSWORD: [root password]
            MYSQL_DATABASE: wordpress
            MYSQL_USER: wordpress
            MYSQL_PASSWORD: [wp password]
        networks:
            wordpress:

    wordpress:
        image: wordpress:latest
        container_name: wp_web
        depends_on:
            - db
        ports:
            - 8000:80
        restart: unless-stopped
        environment:
            WORDPRESS_DB_HOST: db:3306
            WORDPRESS_DB_USER: wordpress
            WORDPRESS_DB_NAME: wordpress
            WORDPRESS_DB_PASSWORD: [wp password]
        volumes:
            - ./data/wp_content:/var/www/html/wp-content
            - ./config/wordpress/uploads.ini:/usr/local/etc/php/conf.d/uploads.ini
networks:
  wordpress:

Create a new directory and save the above file inside it as docker-compose.yml

mkdir wordpress
vi docker-compose.yml

Run docker-compose up -d to fetch the images from the docker hub and create your WordPress instance. You’ll be able to access WordPress from a browser on port 8000.


List of Bank Holidays For England in SQL Format

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First off let’s create a table to store the bank holiday values. You may need to adjust this slightly depending on your SQL server technology being used (this was tested on MySQL/ MariaDB Server) but this is standard SQL dialect and should work on any RDBMS that respects the current SQL standards.

DROP TABLE IF EXISTS bank_holidays;
CREATE TABLE bank_holidays (
  holiday_date date NULL
, holiday_description INT NULL
, PRIMARY KEY (holiday_date)
);

The next step is to insert the bank holiday values below. This table is currently for 2012 up to 2019 for England and Wales.

INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190419', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190422', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190506', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190527', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20190826', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20191225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20191226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20181226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20181225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180827', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180528', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180507', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180402', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180330', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20180101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20171226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20171225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170828', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170529', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170501', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170417', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170414', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20170102', 'New Year’s Day (substitute day)');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20161227', 'Christmas Day (substitute day)');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20161226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160829', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160530', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160502', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160328', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160325', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20160101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20151228', 'Boxing Day (substitute day)');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20151225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150831', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150525', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150504', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150406', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150403', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20150101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20141226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20141225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140825', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140526', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140505', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140421', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140418', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20140101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20131226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20131225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130826', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130527', 'Spring bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130506', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130401', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130329', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20130101', 'New Year’s Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20121226', 'Boxing Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20121225', 'Christmas Day');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120827', 'Summer bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120605', 'Queen’s Diamond Jubilee (extra bank holiday)');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120604', 'Spring bank holiday (substitute day)');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120507', 'Early May bank holiday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120409', 'Easter Monday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120406', 'Good Friday');
INSERT INTO bank_holidays(holiday_date, holiday_description) VALUES('20120102', 'New Year’s Day (substitute day)');

For a more up to date list, please see my github page.

Please submit a PR if you have collated a list yourself, or have any updates to an existing list. The repository currently covers the UK, but I’d be more than happy to accept a PR for other countries! 


MySQL/ MariaDB Error Code: 1329. No data – zero rows fetched, selected, or processed

Category : How-to

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The above error can occur when calling a cursor results in no rows, or no more rows if called in a loop. Whilst the error message is descriptive about what has happened physically, you may wish to catch the error so that you can do something else, or simply replace the generic database error with something more meaningful. 

If you’re not sure what I’m talking about, run the following code on a MySQL or MariaDB database:

DROP PROCEDURE IF EXISTS test_error;

DELIMITER $$
CREATE PROCEDURE test_error() 
BEGIN 
    DECLARE temp_column_name VARCHAR(100);
        
	DECLARE c_example CURSOR FOR 
        SELECT   column_name
        FROM     information_schema.columns
        WHERE    column_name != column_name;
    -- Get data from example cursor
    OPEN c_example;
    FETCH c_example INTO temp_column_name;
    CLOSE c_example;
  
END$$
DELIMITER ;

CALL test_error();

The response is:

Error Code: 1329. No data - zero rows fetched, selected, or processed

In order to trap the error we need to define a CONTINUE HANDLER and DECLARE a variable for the CONTINUE HANDLER to set. We can then manage the No data exception simply by checking the variable.

Following on from the above example, we’ve introduced a variable ch_done. When this variable is set to 1 then the last cursor to be FETCHed returned No data. If it returns a zero then data was returned and all is well.

DROP PROCEDURE IF EXISTS test_error;

DELIMITER $$
CREATE PROCEDURE test_error() 
BEGIN 
    DECLARE ch_done INT DEFAULT 0;
    DECLARE temp_column_name VARCHAR(100);
        
	DECLARE c_example CURSOR FOR 
        SELECT   column_name
        FROM     information_schema.columns
        WHERE    column_name != column_name;

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET ch_done = 1;
    
    -- Get data from example cursor
    OPEN c_example;
    FETCH c_example INTO temp_column_name;
    CLOSE c_example;
    
    IF(ch_done = 1) THEN 
        -- handle the No data error!
        SELECT 'Oh no!';
    END IF;
  
END$$
DELIMITER ;

CALL test_error();

Remember, if you call multiple cursors in a row, you may need to reset the ch_done back to 0.


How to exit from a MySQL/ MariaDB Stored Procedure/ Function Prematurely

Category : How-to

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MySQL and MariaDB enable you to define your own error conditions and to report back to the SQL client both a return code and an error message. As soon as you raise the condition then MySQL/ MariaDB will halt any further execution of the code and report the error back to the client. This can help the user calling the function understand what went wrong, rather than seeing a generic database error message.

DECLARE error_flag INT DEFAULT 0;
DECLARE REF_MISSING CONDITION FOR SQLSTATE '45000';

-- Your code, set the error_flag in the event of an error

IF (error_flag) THEN
    SIGNAL REF_MISSING
    SET MESSAGE_TEXT = 'An error occurred!';
END IF;

The above code defines a custom condition with an error code of 45000 which is the suggested user defined error code. Other error codes are available, which you may have seen, but are reserved by the database server to use for specific database error events – it’s best not to mix your user defined messages with these. The function then checks if the error_flag has been set and, if it has, halts further code execution and returns the error “An error occurred!” to the client.

View from MySQL Workbench

You could simplify this by just calling the below code at the point the error is detected, if you are already catching an error event in your stored procedure or function by simply using the below code without the error_flag declaration.

SIGNAL REF_MISSING
SET MESSAGE_TEXT = 'An error occurred!';


View Table Sizes in MySQL/ MariaDB Databases

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MySQL and MariaDB present a bunch of queryable objects that give you all sorts of insights into what’s happening with the database. 

The size of data stored in tabels is one such thing that can be easily queried directly in SQL, providing you have SELECT access to the information_schema. 

Create the two below views in a schema of your choice:

CREATE  OR REPLACE VIEW schema_size AS
SELECT   table_schema
,        round(SUM(((data_length + index_length) / 1024 / 1024)), 2) table_size_mb
FROM     information_schema.tables  
WHERE table_schema IN ('dv', 'da', 'hue')  
GROUP BY table_schema
ORDER BY table_size_mb DESC;

CREATE  OR REPLACE VIEW table_size AS
SELECT   table_schema
,        table_name 
,        round(SUM(((data_length + index_length) / 1024 / 1024)), 2) table_size_mb
FROM     information_schema.tables  
WHERE table_schema IN ('dv', 'da', 'hue')  
GROUP BY table_schema
,        table_name 
ORDER BY table_size_mb DESC;

See schema_size and table_size for more info.

You can now query each view to get the size, in megabytes, of a whole schema or individual table respectivley. 

SELECT * 
FROM   schema_size;

SELECT * 
FROM   table_size;
  • schema_size – displays the aggregated size of each schema in the database.
  • table_size – lists all tables in the database. You can add a filter to the table_schema column to limit the output.

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