Difference between revisions of "Difference MySQL PostGreSQL"

Revision as of 10:35, 19 June 2014

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Introduction

IMSMA^NG version 6.0 is building on a PostgreSQL database management system, whereas previous IMSMA^NG versions were built on a MySQL database. Both systems are relational database systems and using SQL, the Structured Query Language, as means for interacting with it. However, there are some differences between the MySQL and PosrgreSQL syntax. It is not possible to list all differences here, but the main ones encountered in the IMSMA^NG world are highlighted below. The biggest differences are:

• PostgreSQL is case sensitive
• Built-in functions (e.g. conditional expressions)
• Dates and time formats and queries
• Custom functions

For more details about the PostgreSQL syntax it is advised to consult the official PostgreSQL documentation at [1].

Changes in database views from MySQL to PostreSQL

When upgrading from IMSMA^NG 5.08.04 to 6.0, the database views need to be adapted, i.e. changed from MySQL syntax to PostgreSQL syntax. The country focal point is taking care of this. For reference, this section describes the main changes encountered. This can also be used as a tutorial for creating views in PostgreSQL, for IMSMA^NG administrators who were used to creating them in MySQL.

Case sensitivity and column names

PostgreSQL, as opposed to MySQL, is case-sensitive. This means that strings need to be provided in exactly the way they are stored in the database. For example, in the table IMSMAENUM, one out of many values in the column ENUMVALUE is Progress Report, with capital P and capital R. In MySQL it was possible to write:

SELECT *
FROM imsmaenum
WHERE enumvalue = 'progress report'

and still get one row as result. In PostgreSQL, the same query will return no result.

Therefore, a create view statement like this:
MySQL

CREATE OR REPLACE VIEW my_view AS
SELECT ...
FROM imsmaenum, ...
WHERE imsmaenum.enumvalue = 'progress report';

needs to be changed to:
PostgreSQL

CREATE OR REPLACE VIEW my_view AS
SELECT ...
FROM imsmaenum, ...
WHERE imsmaenum.enumvalue = 'Progress Report';

Similarly, in MySQL it is possible to define column names with a mix of upper and lower case as well as blanks and other special characters without any trouble. In PostgreSQL, this is also possible, but in this case the column name must be enclosed in double quotes. For example, the following create view statement:
MySQL

CREATE VIEW my_view AS
SELECT hazreducdeviceinfo.hazreduc_guid AS hazreduc_guid,
ordnance.model AS Device Type
FROM hazreducdeviceinfo;

needs to be changed to:
PostgreSQL

CREATE VIEW my_view AS
SELECT hazreducdeviceinfo.hazreduc_guid AS hazreduc_guid,
ordnance.model AS "Device Type"
FROM hazreducdeviceinfo;

This also applies to querying the data. For select rows where the device type is equal to 'xy', the syntax is the following: ... WHERE "Device Type" = 'xy'

If an column name in PostgreSQL is written with upper case but not enclosed into double quotes, it will be created in lower case in the database.

CAST AS CHAR CHARSET utf8 and _utf8 (mySQL)

In some (older) MySQL versions, UTF-8 was not the default character set and strings thus needed to be explicitly converted to UTF-8. This is not the case in PostgreSQL and UTF-8 related syntax as used in MySQL does not work in PostgreSQL. Below are two examples encountered in IMSMA^NG views.

MySQL

AND (`customdefinedfield`.`label` = _utf8'CDF-Area Reduction MDU')

PostgreSQL

AND (customdefinedfield.label = 'CDF-Area Reduction MDU')

MySQL

cast(`geopoint`.`latitude` as char charset utf8) AS `lat`

PostgreSQL

cast(geopoint.latitude as char) AS lat

IF and IFNULL (mySQL) / CASE (PostgreSQL) statements (conditional expressions)

The statements IF() and IFNULL() do not exist in PostgreSQL. In PostgreSQL, the CASE statement is used for conditional expressions. The syntax of the CASE statement is as follows:

CASE WHEN condition THEN result
     [WHEN ...]
     [ELSE result]
END

Therefore, for example the following create view statement:
MySQL

CREATE VIEW `my_view` AS
SELECT
sum(if((`my_view2`.`Device Type` = _utf8'SAA'),`my_view2`.`Quantity`,0)) AS`SAA`,
sum(if((`my_view2`.`Device Type` = _utf8'AP'),`my_view2`.`Quantity`,0)) AS `AP`,
sum(if((`my_view2`.`Device Type` = _utf8'AIED'),`my_view2`.`Quantity`,0)) AS `AIED`,
sum(if((`my_view2`.`Device Type` = _utf8'UXO'),`my_view2`.`Quantity`,0)) AS `UXO`,
sum(if((`my_view2`.`Device Type` = _utf8'AT'),`my_view2`.`Quantity`,0)) AS `AT`,
sum(if((`my_view2`.`Device Type` = _utf8'Fragments'),`my_view2`.`Quantity`,0)) AS `Fragments`
FROM
`my_view2`;

needs to be changed to:
PostgreSQL

CREATE VIEW my_view AS
SELECT
sum(case when my_view2.Device Type = 'SAA') then my_view2.Quantity else 0 end) AS "SAA",
sum(case when my_view2.Device Type = 'AP') then my_view2.Quantity else 0 end) AS "AP",
sum(case when my_view2.Device Type = 'AIED') then my_view2.Quantity else 0 end) AS "AIED",
sum(case when my_view2.Device Type = 'UXO') then my_view2.Quantity else 0 end) AS "UXO",
sum(case when my_view2.Device Type = 'AT') then my_view2.Quantity else 0 end) AS "AT",
sum(case when my_view2.Device Type = 'Fragments') then my_view2.Quantity else 0 end) AS "Fragments"
FROM
my_view2;

The ISNULL() function can also be translated into a CASE statement:

CASE WHEN field IS NULL THEN result
END

Date manipulations

There are differences between MySQL and PostgreSQL when it comes to manipulating fields of type DATE, such as for example extracting the year or the month of a date. In MySQL, the related functions are year() and month(). In PostgreSQL, there is an extract() function that does the job. See the below example for reference. The following create view statement:
MySQL

CREATE VIEW `my_view` AS
SELECT
(year(`my_view2`.`DateofAccident`) - year(`my_view2`.`DateofBirth`)) AS `Age`,
year(`my_view2`.`DateofAccident`) AS `Year`,
month(`my_view2`.`DateofAccident`) AS `Month`
FROM `my_view2`;

needs to be changed to:
PostgreSQL

CREATE VIEW my_view AS
SELECT
extract(year from my_view2."DateofAccident") - extract(year from my_view2."DateofBirth") AS "Age",
extract(year from my_view2."DateofAccident") AS "Year",
extract(month from my_view2."DateofAccident") AS "Month"
FROM my_view2;

The GROUP BY clause

References

• PostgreSQL 1.9.4 documentation: http://www.postgresql.org/docs/9.1/static/index.html