DbSchema Tutorial with Introduction in SQL Databases

Contents Connect to the Database and Create First Tables ................................................................................... 2

Create Foreign Keys ............................................................................................................................. 7

Create Indexes ..................................................................................................................................... 9

Generate Random Data ......................................................................................................................... 11

Relational Data Browse ......................................................................................................................... 11

DbSchema Project File ........................................................................................................................... 14

Schema Synchronization ....................................................................................................................... 15

DbSchema Layouts ................................................................................................................................ 16

Visual Query Builder .............................................................................................................................. 17

The SQL Editor ....................................................................................................................................... 18

Forms and Reports ................................................................................................................................ 19

Technical Support .................................................................................................................................. 20

End ......................................................................................................................................................... 20

In this tutorial you will get an introduction into relational databases. Among the tutorial we will use

the DbSchema designer tool. Download, install and open DbSchema from

http://www.dbschema.com. DbSchema can be used on trial base 15 days for free.

Connect to the Database and Create First Tables

Start DbSchema and you should get this screen. Choose ‘Connect to Database’.

We are going to connect to a H2 database. We will use this because H2 requires no other software to

be installed. For MySql, MariaDb, Postgresql, etc., the database software should be installed as well.

If you wish to do this please check in DbSchema Help, in the ‘Databases and How to Connect’ there

are some tips, they will help to install the database software end enable network connections so you

can connect DbSchema to the database.

Choose this

H2 connection to file database

Press ‚new‘ to create a new database in a folder

In the connection dialog choose first ‘H2’ database. This will prompt to download the driver. Choose

‘Download from DbSchema’.

Press ‘Create New’ to create an empty database. In the next dialog you can choose or create an

empty folder.

Press ‘Ok’ in the connection dialog.

Next step is to choose a schema you will work on. A schema is similar with a group of the tables, so

different applications may use the same database but different schemes. ‘Information_schema’ is an

internal H2 schema, with tables used by H2 itself.

Press ok and we should get a screen like bellow. The database is empty, no table is created. Create a

table by right-clicking the layout (the large free space) and ‘Create table’.

Create new folder

In our tutorial we are going to create two tables: Customers and Employees. Each table will have few

columns.

Customers

Customerid – a number, to identify each customer. For example a customer ‘Targot’ will have

the customerid 100, and each time we reffer the customerid 100 we know we talk about this

customer. Using this Id’s has the advantage that we can easy rename the customer and all

other tables which point to it won’t be changed.

Name – some text

Description - text

Employees

EmployeeId – similar with customerid, a number to identify each employee

Firstname – text

Lastname – test

Birthdate – date

Companyid – the id of the company he or she works for

In the table dialog enter the first table name, than add one column at a time by pressing the ‘add’

button.

Right-click the layout and choose ‘Create table’.

When you will enter the table name as well as the column names, you will notice the text is

automatically converted to upper-cases. This because the database is using by default uppercases for

table and column names, and DbSchema convert them to show them as they are looking in the

database.

Let’s add the first column CUSTOMERID. Enter the name, choose the data type ‘Integer’ which means

number. The checkbox ‘Primary Key’ should be on, which means this column will unique identify any

record from this table. Primary keys must always be specified. This means all records should have a

value in this field and therefore the mandatory field is checked and cannot be un-checked.

Enter the table name

Add columns one by one

Similar we add the name and description as varchar (this is text) with maximal number of characters

allowed 100 for name and 500 for description. The name should be mandatory in order to make sure

no company is missing the name.

After adding all columns the table will look like here:

Or like this:

Primary key and ‘x’ for mandatory symbols

Press this button to see the data type

Similar create the second table EMPLOYEES.

Create Foreign Keys

To prevent having employees with CUSTOMERID values which does not exists in the customer table,

we are going to create a foreign key. The foreign key is a ‘constraint’, which means it is a logical

verification of the data. This verification takes place each time a new record is inserted in the table or

the data is modified.

To create the foreign key, drag and drop the CUSTOMERID from EMPLOYEES over the CUSTOMERID

in CUSTOMERS. We want that each EMPLOYEE.CUSTOMERID exists in the CUSTOMERS.CUSTOMERID.

The foreign key dialog will open:

Drag & drop column

The foreign key requires the referring table (employees) and referred table (customers) plus the

columns for each of them. We can set an action to be taken if a customer is deleted. No action

means nothing will be done. If we will try to drop a customer which has employees we will get back

an error message. We can set ‘on delete cascade’ and all employees belonging to a customer will be

deleted when the company is deleted.

‘Virtual’ means the foreign key is created only in DbSchema. In this case no data verification will be

done in the database. The virtual foreign keys may be used only for design purposes or for relational

data browse which will be explained later.

A foreign key always require an index to exist on the referred columns (CUSTOMERID in customers

table). If this column is a primary key will automatically get an index. Read more about indexes in the

index chapter.

About the foreign key impact on performance

Is a common mistake to skip creating foreign keys because they slow down the database. This is not

true. The foreign keys are a validation of the data which is done only when inserting or deleting new

rows. Usually the ID fields don’t get modified, so the foreign key won’t be triggered. The foreign key

validation won’t be done when updating other columns than the foreign key columns.

Foreign keys are useful as they will prevent having bad data in the database. This is very important!

Foreign keys are healthy!

Create Indexes

Database indexes are used to fasten searching of data by a given criteria. Consider a query SELECT *

FROM EMPLOYEES WHERE FIRSNAME=’JOHN’. Without indexes the database will scan the complete

data before finding the employees with this firstname. This may take time if the table contains many

records. Having an index on FIRSTNAME will fasten the search.

Indexes work similar with a book keyword index, which can be usually found in the end of a book. An

index is a list of values in a column with a reference to the position where the values can be found in

the table.

Indexes duplicate the column data in a separate structure and hold for each distinct value a list of

pointers to the table records. The index is synchronized by the database server each time the column

data is modified, inserted or deleted.

Let’s create an index on EMPLOYEES.FIRSTNAME. Right-click the FIRSTNAME column and choose ‘Add

Index’.

An index can be normal, which means any value can be stored inside. Unique indexes do not allow

duplicate values. When a value is inserted in the table field and that value exists in another record for

the same field, the insert operation will throw an error and the operation is aborted.

Primary Key indexes are the same as unique indexes plus the condition that the column must be

mandatory (does not allow missing or empty values). A table can have at most one primary key, but if

you need more you can create unique indexes and set the columns to be mandatory. This has the

same effect as creating a primary key column.

After creating the index a symbol will show in front of the column.

Indexes require more space in the database and decrease the database performance on inserts and

updates, but increase the performance by selects.

In case of foreign keys with ‘on delete cascade’ is recommended to set-up an index on the referring

column ( customerid in employees ). This will fasten the deletion of any customer from customer

table.

Queries can use at most one index for a table. If for example you want to search an employee by

firstname and lastname and you have one index for firstname and another index for lastname, only

one index will be used. One solution is to create a compound index on two or more columns. Create

an index on firstname and lastname in the same time. This will help on searching by firstname and

lastname, or only by firstname. The index won’t help searching only on lastname - either the first or

Index symbol

first and second columns have to be specified in the search criteria. Having a value only for the

second column won’t use the index.

Generate Random Data

Let’s fill the tables with some random data. For this choose the random data generator from the

menu.

In the dialog press ‘Generate’ to start the generator. Optional we can configure other patterns for

the fields as the default settings. Double-click one of the tables to open the patterns editor for each

of the columns.

Relational Data Browse

DbSchema has a dedicated tool for exploring and editing the data. Is called ‘relational’ because can

explore data from multiple tables based on the foreign keys between them.

Generate data

Generate ramdom data

Right-click on CUSTOMERS table header and press the Relational Data Browse from the popup. The

option is available in the right-click pop-up as well.

The editor will open. We can descend in the EMPLOYEES table by pressing the foreign key arrow in

the table header. Choose ‘Join EMPLOYEES’.

Now we are browsing two tables. Select a different record in the first editor will refresh the second

with the matching rows based on foreign key, which are the matching employees for the selected

company.

Relational Data Browse

Descend into EMPLOYEES

To filter a column data click the column header.

To edit the data double-click any cell.

Add or delete row

Double-click cell to edit.

DbSchema Project File

Our work including the schema, diagrams, SQL Editors and relational data browse can be saved to

project file. The file has the extension .dbs.

Next time when you restart DbSchema, the project file will be reopened. If you are working with a

dedicated database server and you are in a location where you cannot connect to the database, the

project file can still be opened and the diagrams will show. The relational data browse cannot be

used because the data has to be loaded from the database.

Save project to file.

Schema Synchronization

DbSchema is using its own image of the schema. This is what is being saved in the project file.

Whenever you connect to the database you have to press the refresh button to make sure the

internal schema is the same as in the database.

DbSchema can be used online (connected to database) or offline. Switching to online will require

refreshing schema from the database or creating schema in the database (if you designed a fresh

schema directly in DbSchema offline). After refresh you may decide which differences to take over in

DbSchema internal project or apply in the database.

If you design a schema offline you can connect to any database and create the schema in the

database. You may improve an existing schema, connect and deploy the latest changes on different

databases.

Refresh schema

Online or offline

DbSchema Layouts

A layout is the diagram and the associated tools opened inside ( Relational data browse, SQL editor,

Query builder, Database reports, etc. ). Create further layouts from the menu.

One table may show in different layouts or in none of them. Adding tables to layout can be done by

drag & drop or by pressing the foreign key icon beside columns.

Create further layouts

Drag & drop tables to layout from here

Add further tables to layout by pressing the foreign key icon

Visual Query Builder

Relational data browse is one tool for accessing the data. Beside it we can use the SQL Editor and the

Query builder. Start the query builder by clicking the table header.

The builder will open with the selected table. We are going to build a query over both tables. We add

the CUSTOMERS table by clicking the foreign key icon.

Tick the checkboxes for the columns we want to show in the result. To set a filter (where clause) right

click a column.

Execute the query. The result can be saved to file. The generated query is on the left, in SQL Preview

frame.

Foreign key icon

The SQL Editor

Use the SQL editor will to edit and execute SQL queries. Text auto-completion is enabled on CTRL-

BLANK keys.

Execute the query

Generated query

Forms and Reports

Using DbSchema you can create small applications or database reports. Start a report from the

application menu, edit the query in the query builder (called data source, add tables to it by drag &

drop).

One more screen to select the components to add to report:

…and the report is ready to be executed from the menu.

After editing and executing the query the wizard will continue.

Execute the report and the web browser will show the data.

On DbSchema website there are dedicated tutorials for learning forms and reports. Read them and

you will discover how easy is to create a master-detail report, do customization, etc.

Technical Support

On DbSchema website there are contact forms where you can write us with any technical questions.

Unregistered users may ask as well. Don’t hesitate to write!

End

Thank you for taking this course. Please give us feedback about DbSchema, your help is appreciated!

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