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Quick Lesson on Databases

Quick Lesson on Databases. Relational databases are key to managing complex data You’ve been using relational databases with “Joins” and “Relates” in ArcGIS GeoDatabases are relational databases Structured Query Language (SQL) is the primary language for relational databases

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Quick Lesson on Databases

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  1. Quick Lesson on Databases • Relational databases are key to managing complex data • You’ve been using relational databases with “Joins” and “Relates” in ArcGIS • GeoDatabases are relational databases • Structured Query Language (SQL) is the primary language for relational databases • You’ve been using SQL statements in ArcGIS to query data

  2. Relational Databases • Need to represent data with a complex structure Plot Species Tree

  3. Database Tables • What you’ve seen in ArcGIS only more flexible • Tables are made up of “fields” (columns) and “records” (rows) • Queries are used to combine and subset tables into new tables • Each table should have a unique, integer, ID, referred to as a primary key • Greatly improves query performance

  4. Field Data Types • Numeric • Float or integer • Auto numbered, use for primary keys • Dates • YYYY-MM-DD HH:MM:SS.SS • 2013-04-05 14:23:12.34 • Text • Specified width • “Variant” width • Binary Large Objects (BLOB)

  5. What’s Wrong With This?

  6. Relational Databases • Allow us to “relate” tables to: • Reduce the overall amount of data • Removes duplicates • Makes updates much easier • Improves search speeds

  7. Entity-Relationship Diagram • ERD • Unified Markup Language (UML) Relationship Types One to one One to many Many to many Entities Plot Relationships Species Tree

  8. Plot Tree Species Primary Key Foreign Key

  9. Database Normalization • Eliminate duplicate columns from the same table • Move fields that have “duplicate” row entries and move them to a related table • All field entries should be dependent on the primary key • There should be only one primary key in each table

  10. Database Dictionary • Defines each of the tables and fields in a database • A database forms the basis for data management behind many GIS projects, web sites, and organizations • Proper documentation is key to long term success! • Database design (including ERDs) • Database Dictionary

  11. Geospatial Databases • Not required to store spatial data! • Provide: • Field types for spatial data: point, polyline, polygon, etc. • Spatial operations: union, intersect, etc. • Spatial queries: return records that overlap with a polygon, etc. • Some provide spatial reference control

  12. Relational Databases • Enterprise-Level • SQL Server • PostgreSQL • MySQL • Oracle • Sybase • File-Level • Geodatabase • MS-Access

  13. What we really want • What we need from a database: • Distributed, concurrent access (concurrency) • Automatic Backup • Version control • Unlimited amounts of data • Quick data access • Inexpensive • Broad OS Support • File-level copying • GeoSpatial queries, operations, data types

  14. What we have

  15. Structured Query Language (SQL) • Comes from the database industry • “INSERT”, “DELETE”, and “SELECT” rows in tables • Very rich syntax • Portions of “SELECT” grammar used heavily in ArcGIS: • Selecting attributes • Raster calculator • Geodatabases

  16. Transaction SQL • “SQL” is a subset of T-SQL • T-SQL allows full management of a database: • Create & drop: • Tables, fields/columns, relationships, indexes, views, etc. • Administrative functions • Varies some between databases

  17. Using SQL • All Databases have “query editors” that allow us to write, save, edit, and use SQL queries • Use programming languages to “write” queries and “fetch” records from the database

  18. SQL: SELECT SELECT Field1, Field2 FROM TableName JOIN TableName2 WHERE Filter1 AND Filter 2 GROUP BY Field1,Field2 ORDER BY Field1 [DESC], Field2 [DESC]

  19. Selecting Fields • SELECT * • Returns all fields as new table • SELECT Field1,Field2 • SELECT Table1.Field1,Table2.Field1 • Return specified fields • SELECT Table1.Field1 AS NewName • Avoids name collisions

  20. Selecting Tables • FROM Table1 • Returns contents of one table • FROM Table1 INNER JOIN Table2 ON Table2.ForeignKey=Table1.PrimaryKey • Returns records from Table2 that match primary keys in Table1 • Does not return all rows in Table1

  21. Selecting Tables (con’t) • FROM Table1 OUTER JOIN Table2 ON Table2.ForeignKey=Table1.PrimaryKey • Returns all matches between Table1 and Table2 and any records in Table1 that don’t match records in Table2 • Missing values are NULL

  22. Filters or “WHERE” clauses SELECT * FROM Table1 WHERE (Field1 Operator Value1) BooleanOperator (Field1 Operator Field2)

  23. Filter Examples • WHERE: • ID = 1 • Area < 10000 • Area <= 10000 • Name = “Crater Lake” (case dependent) • Name LIKE “Crater Lake” (ignores case) • Notice: • String values have double quotes • Syntax for strings vary some between databases

  24. SQL Comparisons • Equals: = • Greater than: > • Less than: < • Greater than or equal: >= • Less than or equal: <= • Not equal: <> • Like: case independent string comparison with wild cards (%)

  25. Boolean Operators

  26. More Complex Filter Examples • WHERE: • Name LIKE “Hawaii” AND Area < 10000 • Species LIKE “Ponderosa” AND DBH > 1

  27. ORDER BY SELECT * FROM Table 1 ORDER BY LastName DESC, FirstName DESC • Careful with performance on large datasets and string fields

  28. GROUP BY • Aggregates data SELECT Species ,AVG(Height) FROM Trees GROUP BY Species • Only aggregated fields can appear in SELECT list

  29. SQL INSERT • INSERT INTO TableName (Field1,Field2) VALUES (Value1,”Value2”) • String values must be in quotes • Other values can also be in quotes • If the table has an “auto numbered” ID field, it will be added automatically • Otherwise, very difficult to set the ID field

  30. SQL DELETE DELETE FROM TableName WHERE ID=Value - Deletes one row DELETE FROM Plot WHERE PlotID=12 - Deletes all rows with PlotID=12 DELETE FROM TableName - Deletes everything in TableName!

  31. Database Performance Default Search Indexed Search Primary Key Search

  32. Indexes • Added to a table • Typically for one field • Adds overhead to INSERT and DELETEs • Important for: • Large tables • Complex queries • Especially text searches!

  33. Maintaining Performance • Always use integer, auto numbered primary keys • Avoid iterative or hierarchical queries • Sometimes code is faster: • Do simple query, load into RAM and sort • With REALLY big data, don’t use SQL • NoSQL, accessing data directly, without the use of a relational database package • There are “NoSQL” products in the works • Avoid text searches and sorts

  34. Rasters and Databases • Don’t put rasters into a database! • Makes it impossible to backup and restore the database • Put a file path to the rasters in the database

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