Heuristic optimization often includes making transformations to the query tree by moving operators up and down the tree so that the transformed tree is equivalent to the tree before the transformations. Before we discuss these heuristics, it is necessary to discuss the following rules governing the manipulation of relational algebraic expressions: 1. Joins and Products … Read more
Before query optimization is carried out, one would of course need to decide what needs to be optimized. The goal of achieving efficiency itself may be different in different situations. For example, one may wish to minimize the processing time but in many situations one would wish to minimize the response time. In other situations, … Read more
In most database systems, queries are posed in a non-procedural language like SQL and as we have noted earlier such queries do not involve any reference to access paths or the order of evaluation of operations. The query processing of such queries by a DBMS usually involves the following four phases: 1. Parsing 2. Optimization … Read more
SQL language also includes syntax to update, insert, and delete records. These query and update commands together form the Data Manipulation Language (DML) part of SQL: • INSERT INTO – inserts new data into a database table • UPDATE – updates data in a database table • DELETE – deletes data from a database table … Read more
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables. The most important DDL statements in SQL are: • CREATE TABLE – creates a new database table • ALTER TABLE – alters (changes) … Read more
Here you can see that SQL commands follow a number of basic rules: • SQL keywords are not normally case sensitive, though this in this tutorial all commands (SELECT, UPDATE etc) are upper-cased. • Variable and parameter names are displayed here as lower-case. • New-line characters are ignored in SQL, so a command may be … Read more
There are three groups of commands in SQL: 1. Data Definition 2. Data Manipulation and 3. Transaction Control
Structured Query Language, commonly abbreviated to SQL and pronounced as “sequel”, is not a conventional computer programming language in the normal sense of the phrase. It allows users to access data in relational database management systems. SQL is about data and results, each SQL statement returns a result, whether that result be a query, an … Read more
Once entities and relationships have been identified and defined, the first draft of the entity relationship diagram can be created. This post introduces the ER diagram by demonstrating how to diagram binary relationships. Recursive relationships are also shown. Binary Relationships Figure 1 shows examples of how to diagram one-to-one, one-to-many, and many-to- many relationships. Figure … Read more
The data model is one part of the conceptual design process. The other is the function model. The data model focuses on what data should be stored in the database while the function model deals with how the data is processed. To put this in the context of the relational database, the data model is … Read more