Postgres Nuts and BoltsThis module explores the building blocks of a Postgres database in greater detail. Data HierarchySchemas are the equivalent of folders in a file system, supporting the organisation of other database objects into logical groups (for example planning, education, highways). All tables must be inside a schema, and schemas also hold function and views. Within a schema, tables store the records or rows which make up your data, and have rows and columns like a spreadsheet. Tables have columns which define the structure of the data stored in a table, including name, datatype (string, integer, date etc) and size. Create Schema and Table Create a new schema in pgAdmin by navigating in your database to Schemas, then right-click > Create > Schema… Give the schema a name (for example training) and keep the owner as postgis, then click Save to create it Now recreate the structure of an existing table in the new schema Navigate to crime > neighbourhood_crime, and click SQL in the toolbar at the top of the window - you will see the SQL needed to create the table on the right
Copy the SQL from the window on the right, and click the Query Tool button in the toolbar, then paste the SQL into the Query window Delete the last four statements (TABLESPACE, ALTER TABLE, GRANT ALL) Edit the references to the crime schema in the SQL to reference your new schema name, then click the Execute/Refresh (Play) button in the toolbar to create the new table Click on the new table in the new schema, and click the View Data button in the toolbar In the Data Output window at the bottom, click the Add Row button, then double-click on a cell to add data - try both valid and invalid data to see the result When you are done, click the Save Data Changes button to commit your changes You now have a new table with a row of data
In the next exercise, we’ll look in more detail at a table, the objects within it, and what can be done with it how to manage them in pgAdmin. Columns Constraints include requirements for records in a table - for example the primary key, or the geometry type the data must representAdd a column Navigate to crime > street_crime and open the Columns node You will see that there is a wkb_geometry column - click on this and look at the Properties tab for it on the right - the data type is geometry, meaning that it is a spatial table and so can be used in GIS applications and analysed spatially Right-click on street_crime > View/Edit Data > All Rows to examine the data In QGIS, navigate to the same table under your PostgreSQL node in the Browser, and double-click to load it into QGIS Right-click > Open Attribute Table on the new layer - you will see the same data as you saw in pgAdmin In pgAdmin, click on Columns > right-click > Create > Column Create a new column called officer - this will hold the name of the officer who dealt with the crime In the Definition tab, set: Data type: character varying Length/Precision: 200
Click Save to add the column Open QGIS, right-click > Refresh on the connection and load the table again - you should see the new column
Indexes are included to speed up search and retrieval of data Triggers define actions to be taken when an event happens - for example when a new row is added to a table Functions are equivalent to stored procedures - they are canned SQL which performs a task Sequences implement auto-numbering Views are virtual tables which are the result of SQL applied to one or more real tables, and ordinary views can not be edited
For a database cluster: Tables Datatypes I Goal of Exercise Do this Do something else - this is what you get
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