SQL
External
- Learning SQL, 3rd Edition by Alan Beaulieu
- http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
Internal
Overview
This article documents standard SQL. For database-specific extensions, we'll link to the databases-specific DML operations page.
SQL is a standard non-procedural language designed to work with relational databases, allowing the users to interact with the database instance by issuing schema statements, data statements and transaction statements.
SQL statements define the necessary inputs and outputs for the database interaction, but it does not define how the statements should be executed. The details of the execution are left to the database engine component known as the optimizer. The optimizer parses the SQL statements and depending on how the tables are configured and what indexes exist, it decides the most efficient execution path. Most database engines will allow you to influence the optimizer's decisions by accepting optimizer hints.
The first standard for the SQL language was published by the American National Standards Institute (ANSI) in 1986. Subsequent refinements led to new releases of the SQL standard in 1989, 1992, 1999, 2003, 2006, 2008, 2011, and 2016.
Relational Database Terminology
Table
In a relational model, data is stored in a set of tables, also known as relations. Within a relation, data is organized in rows (records) and columns. Each row describes an entity. Each is uniquely identified by a primary key. The rows may contain columns that are copies of other tables' primary keys. These are knowns as foreign keys and are used in joining related data together.
Also see:
From an SQL perspective, there are four types of tables. All of them can be used in the FROM clause of a SELECT query:
Permanent Table
A table created with a CREATE TABLE statement.
Temporary Table
A temporary (or volatile) table can be created with CREATE TEMPORARY TABLE. It looks just like a permanent table, but any data inserted into a temporary table will disappear at some point, generally at the end of a transaction or when the database session is closed.
Derived Table
Derived tables are dynamically built in memory when a containing query executes a subquery. The derived table data is held in memory for the duration of the query execution and then discarded. See:
Virtual Table (View)
A view, or a virtual table, is a query that is stored in the database's data dictionary. It looks and acts like a table, but there is no data associated with the view. A view is created with a CREATE VIEW statement.
Result Set
The result of an SQL query, known as a result set, is also a table maintained in memory.
View
SQL Schema Statements (DDL)
SQL schema statements are used to define the data structures stored in the database. This SQL subset is also known as Data Definition Language (DDL). All database elements (tables, constraints, etc.) create with the SQL schema statements are stored in a special set of table called the data dictionary, or metadata. Also see:
CREATE | ALTER | DROP TABLE
CREATE | ALTER | DROP VIEW
Constraints
SQL Data Statements (DML)
SQL data statements are used to manipulate the data structures perviously defined using the SQL schema statements. This SQL subset is also known as Data Manipulation Language (DML).
Queries with SELECT
INSERT
UPDATE
DELETE
SQL Transaction Statements
The SQL transaction statements are used to begin, end, and rollback transactions.
Standard SQL Data Types
SQL NULL
There are cases when values for certain columns cannot be determined. In these cases, the column is said to be NULL (it is not said that it equals to NULL). NULL indicates an absence of a value. NULL is used in the following cases:
- not applicable
- unknown
- empty set
When creating a table, the columns are allowed to be NULL by default. If specific columns must not accept NULL values, they must be declared with a NOT NULL constraint.
Also see:
Indexes
Set Operations
Analytic Functions
TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch16.html
Parameterized SQL Statements
Parameterized SQL Statements in PostgreSQL Parameterized SQL Statements in MySQL