Last Sync: 2022-08-05 20:00:04

Databases/SQL/2_SELECT.md

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+---
+tags:
+  - Programming_Languages
+  - Databases
+  - sql
+---
+
+# SQL: The SELECT query
+
+## Print/retrieve/write an entire table, unfiltered
+
+````sql
+SELECT * FROM [table_name]
+
+SELECT * FROM model
+````
+
+## Retrieve all data from a specific field
+
+````sql
+SELECT [field_name] FROM [table_name]
+
+SELECT name FROM manufacturer
+````
+
+## Retrieve data and order it
+
+This example orders alphabetically: 
+
+````sql
+SELECT [field_name] FROM [table_name] ORDER BY [property]
+SELECT name FROM model ORDER BY name 
+````
+ > When `ORDER BY` is used the default method for strings is alphabetical and for integers it is ascending order.
+
+Here's a more complex real-life request:
+
+````sql
+SELECT name, cores, ram FROM model ORDER BY ram, name
+````
+
+It gives us:
+
+````
+name              cores       ram
+----------------  ----------  ----------
+Commodore VIC-20  1           0.005
+Commodore 64      1           0.064
+Amiga 500         1           0.5
+Apple Lisa        1           1.0
+Raspberry Pi 1 M  1           256.0
+Raspberry Pi 1 M  1           256.0
+Raspberry Pi 1 M  1           512.0
+Raspberry Pi 1 M  1           512.0
+Raspberry Pi 1 M  1           512.0
+Raspberry Pi Zer  1           512.0
+````
+
+But we can obviously specify our own ordering method:
+
+````sql
+SELECT name, cores, release_date FROM model ORDER BY cores DESC, name;
+````
+
+Returns:
+
+````
+name               cores       release_date
+-----------------  ----------  ------------
+Apple MacBook Pro  6           2019-05-21
+Apple iMac         4           2019-03-19
+Raspberry Pi 2 Mo  4           2015-02-01
+Raspberry Pi 3 Mo  4           2018-11-01
+Raspberry Pi 3 Mo  4           2016-02-01
+Raspberry Pi 3 Mo  4           2018-03-14
+Raspberry Pi 4 Mo  4           2019-06-24
+Raspberry Pi 4 Mo  4           2019-06-24
+Raspberry Pi 4 Mo  4           2019-06-24
+````
+
+ > 
+ > `ORDER BY` always comes last, after the selection and any filtering clauses but *before* a `WHERE` clause

Databases/SQL/5_UPDATE.md

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+---
+tags:
+  - Programming_Languages
+  - Databases
+  - sql
+---
+
+# SQL: The UPDATE clause
+
+With UPDATE we modify existing records.
+
+## Schematic syntax
+
+````sql
+UPDATE [table_name]
+SET [field]
+WHERE [conditional expression/filter]
+````
+
+## Real example
+
+````sql
+UPDATE manufacturer
+SET url = '<http://www.hp.co.uk>'
+WHERE manufacturer_id = 4; --typically this will be the primary key as you are updating and existing record and need to identify it uniquely
+````
+
+## Multiple fields
+
+````sql
+UPDATE manufacturer
+SET url = '<http://www.apple.co.uk>',
+    year_founded = 1977
+WHERE manufacturer_id = 2;
+````

Databases/SQL/6_DELETE.md

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+---
+tags:
+  - Programming_Languages
+  - Databases
+  - sql
+---
+
+# SQL: The DELETE query
+
+````sql
+DELETE FROM sales WHERE sale_id = 1;
+````