eolas/Databases/SQL/1_Language_structure.md

tags
Programming_Languages Databases sql

SQL: Language structure

Before we start using the syntax we need to understand the grammar:

Expressions differ from clauses and predicates in that they are not the mechanism for returning data (i.e. declaring a clause and a logical colllllndition) they do something to the data, as part of the retrieval. This is a bit subtle:

• SELECT name FROM model WHERE cores = "4"
  • This retrieves the models that have 4 cores
• SELECT count(*) FROM model WHERE cores = "4"
  • This counts the number of models that are returned where the counting is a function over and above the retrieval itself.

Examples from computer_sales.db

sqlite> SELECT * from model WHERE cpu_speed=0.7 : return all models with a CPU speed equal to 0.7:

model_id    manufacturer_id  name                    cpu_speed   ram         cores       wifi        release_date
----------  ---------------  ----------------------  ----------  ----------  ----------  ----------  ------------
1           1                Raspberry Pi 1 Model A  0.7         256.0       1           0           2013-02-01
2           1                Raspberry Pi 1 Model B  0.7         256.0       1           0           2012-04-01
3           1                Raspberry Pi 1 Model B  0.7         512.0       1           0           2012-10-01
4           1                Raspberry Pi 1 Model A  0.7         512.0       1           0           2014-11-01
5           1                Raspberry Pi 1 Model B  0.7         512.0       1           0           2014-07-01

count(*)
----------
5

Any value that is not a number should be in single-quotes, never double quotes

Main commands

There are obviously many SQL commands but most standard CRUD actions can be executed with a small number of commands:

• SELECT
• UPDATE
• CREATE
• INSERT
• DELETE