Database. Ed Milne. Theme An introduction to databases Using the Base component of LibreOffice

Transcription

1 Theme An introduction to databases Using the Base component of LibreOffice Database Ed Milne Database A database is a structured set of data held in a computer SQL Structured Query Language (SQL) is a programming language used to create, maintain and extract data from databases It is an ISO standard e.g. SELECT isbn, title, price, price * 0.14 AS sales_tax FROM book WHERE price > 100 ORDER BY title Base Base, a component of the LibreOffice suite is a front end to a database The back end, the database engine, can be A spreadsheet (Calc) HSQL open source freeware embedded in base External sophisticated database engines such as PostgreSQL, MySQL and MariaDB Database Engines The appropriate database engine depends on the performance of your computer and the amount of data in the database As a rough guide Spreadsheets up to 1,000 records HSQL up to 50,000 records MYSQL etc. - over 50,000 records Using HSQL creates a database in a single file Using other engines creates two objects The Base front end The DB engine containing the data This can be useful when more than one person can be accessing the data at the same time In an organization, the engine can be on a server and a copy of the front end is on each employee's computer Sophisticated engines prevent two people from changing the same record at the same time Separate engines also make maintenance easier Changes to the front end can be copied to each user e.g. New forms or reports Changes to the engine only have to be made at a single location - 1 -

2 A separate front end and engine is also used on the web e.g. When you register at a website, the form on the web page is the front end of a database Tables The data in a database is organized in tables which look much like a spreadsheet LibreOffice has a wizard for creating tables Fields A table consists of fields Each field is the equivalent of a column in a spreadsheet Field Data Type Each field can only accept a specific type of data cf. a spreadsheet where you can enter any type of data in any cell Integer Big integer up to 19 digits Integer up to 10 digits Small integer up to 5 digits Tiny integer up to 3 digits Numbers Decimal 10 digits Fixed number of decimal places Float, Real, Double 17 digits e.g. 1.23E+017 Text Text only uses the space required for the data Text (fix) Text (ignore case) Long text (memo) up to 2,147,483,647 characters Date and Time Date Time HH:MM:SS Timestamp date and time Boolean Yes/No Binary - up to 2,147,483,647 bytes Image Binary Binary (fix) Other Up to 2,147,483,647 bytes Indexes Each table must have a primary index to uniquely identify each record cf. The row number in a spreadsheet - 2 -

3 The integer data type has an autonumber feature which automatically creates a unique number for the record Relations You can relate tables together by Adding a field for the primary index in another table to a table Linking these fields together One-to-Many Relationship In the example shown, each media record can be linked to many books Many-to-Many Relationship A many-to-many relationship can be made using an intermediate table In the example shown, a book can have more than one subject and a subject can relate to many books Views You can create a View of a table which contains a subset of the fields in a table a set of fields from related tables A view is a virtual table which can be used much like a table Forms Forms let you add or edit the information in a database LibreOffice has a wizard for creating forms from a table or view The forms editor is the word processing component of LibreOffice After creation, you can modify the form Filters You can filter the data on a form so only records with certain values appear e.g. You can filter the data in a book form so that only records with the medium Magazine appear as you browse through the records Datasheet Form A datasheet form appears like a spreadsheet Subforms Subforms are forms within a form Typically a datasheet form within a field form Form Controls Check box Text box Numeric field Currency field Date field Time field Pattern field Spin button - 3 -

4 Push button Option button Image button (icon) List box Combo box Label Image Scroll bar Frame Navigation bar Check Box A check box accepts a Boolean (Yes/No) value Currency Control Accepts a currency value The spin box control on the right increments or decrements the value by $1 Date and Time Controls There are various ways to format the date and time The large down arrow displays a calendar The spin buttons change the value selected by the position of the cursor E.g year, month or day Dropdown Menus Lets you select a value from a related table A list of constants embedded in the form Dropdown Menus A list box saves the index of the record in the related table in the primary record A combo box saves the displayed text in the primary record Either control can save the selected value in the form rather than the record Buttons Buttons can be used to execute a macro for the form set a value in the record Macros Macros are subroutines in the programming language used by the application In this case OpenOffice Basic You can create macros by Learning the programming language and writing the code Recording a macro Searching the web From books - 4 -

5 Executing Macros You can execute a macro by Using the Run Macro option in the menus Assigning a shortcut key to the macro Linking the macro to an event for a form or control Queries Queries let you select, organize and manipulate data from the database Queries are used to select and organize the fields for dropdown menus reports Base provides a Graphical User Interface (GUI) for select queries to Select fields Determine which fields should be visible Select records with specific values in a field Sort the extracted data Apply functions like Count or Average to the fields You can use other queries as well as tables as the data source i.e. You can created nested queries In Base, you hand code other types of queries in SQL e.g. An Update query to make a mass change to the data UPDATE "tblsubjectwork" SET "SubjectID" = '5' WHERE "SubjectID" = '7' Reports Base has a report generator to create reports from queries or tables Reports can be Grouped Group headers and footers Sorted Output to text documents or spreadsheets Database for Spreadsheets Why use a spreadsheet as the back end of a database? You already have the data in a spreadsheet You want to use some features of a spreadsheet like charts and graphs A database allows strict data typing and limits E.g. A value must be present and within a specific range Forms avoid horizontal scrolling Database reports References LibreOffice Base Guide Base Tutorial - 5 -