basic understanding of SQL and to get a feel of how it works. Before you start practicing with various types of examples given in this . SQL Commands. This tutorial will give you quick start with SQL. Before you start doing practice with various types of examples given in this reference, I'm SQL Commands. again. Tins lesson also covers the use of SQL*Plus commands to execute . example on the slide, displays all columns and all rows of the DEPT table: SELECT.
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Glossary of commonly used SQL commands. Codecademy is the easiest way to learn how to code. It's interactive, fun, and you can do it with your friends. 𝗣𝗗𝗙 | This book offers a short reference tutorial for database Structure of inner and natural joins An example using INNER JOIN syntax is given usually executed through a specific SQL command-line interface (CLI). SQL relation table tuple row attribute column. ▫ Syntax notes: Assigned to column in any tuple for which a value is not specified. ▫ Example. CREATE TABLE.
Query within another query is called as Subquery.
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A subquery is called inner query which returns output that is to be used by another query. Question 24 How many row comparison operators are used while working with a subquery?
Question 25 What is the difference between clustered and non-clustered indexes? One table can have only one clustered index but multiple nonclustered indexes.
Clustered indexes can be read rapidly rather than non-clustered indexes. Question 28 How to write a query to show the details of a student from Students table whose name starts with K?
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Subquery within another subquery is called as Nested Subquery. If the output of a subquery is depending on column values of the parent query table then the query is called Correlated Subquery.
Here are a few of the most important ones — each of these queries is consequential to almost every system that interacts with an SQL database. Tables; This is perhaps the most widely used of SQL queries examples. In addition, it groups the results according to the country of each customer. Query for Creating a View A view is a tailored table that is formed as a result of a query.
It has tables and rows just like any other table. Query for Retrieving a View The standard syntax of selecting attributes from a table is applicable to views as well. Query to Display User Tables A user-defined table is a representation of defined information in a table, and they can be used as arguments for procedures or user-defined functions.
Query to Display Primary Keys A primary key uniquely identifies all values within a table. Displaying Foreign Keys Foreign keys link one table to another — they are attributes in one table which refer to the primary key of another table. The list of defined triggers can be viewed using the following query. Displaying Internal Tables Internal tables are formed as a by-product of a user-action and are usually not accessible. The data in internal tables cannot be manipulated; however, the metadata of the internal tables can be viewed using the following query.
Before we conclude this example, let's run the mysqldump utility program to dump out backup the entire southwind database.
You can restore from the backup by running the script either:. In the relational model, a table shall not contain duplicate rows, because that would create ambiguity in retrieval. To ensure uniqueness, each table should have a column or a set of columns , called primary key , that uniquely identifies every record of the table.
For example, an unique number customerID can be used as the primary key for the customers table; productCode for products table; isbn for books table. A primary key is called a simple key if it is a single column; it is called a composite key if it is made up of several columns.
The primary key is often used to relate to other tables.
A foreign key of a child table is used to reference the parent table. Foreign key constraint can be imposed to ensure so-called referential integrity - values in the child table must be valid values in the parent table.
We define the foreign key when defining the child table, which references a parent table, as follows:. Indexes or Keys can be created on selected column s to facilitate fast search. If productID column is indexed e. The drawback about indexing is cost and space. Building and maintaining indexes require computations and memory spaces.
Nevertheless, relational databases are typically optimized for queries and retrievals, but NOT for updates. There can be more than one indexes in a table.
Index are automatically built on the primary-key column s. Results of one query can be used in another SQL statement. Subquery is useful if more than one tables are involved. In the previous many-to-many product sales example, how to find the suppliers that do not supply any product? A subquery may return a scalar, a single column, a single row, or a table.
You can use comparison operator e. Date and time are of particular interest for database applications.
Let's begin with Date without Time with the following example. Take note that date value must be written as a string in the format of 'yyyy-mm-dd' , e. MySQL provides these built-in functions for getting the current date, time and datetime:.
MySQL will issue a warning and insert all zeros e. A view is a virtual table that contains no physical data. It provide an alternative way to look at the data.
Transaction is important to ensure that there is no partial update to the database, given an atomic of SQL statements. If you start another mysql client and do a SELECT during the transaction before the commit or rollback , you will not see the changes.
Alternatively, you can also disable the so-called autocommit mode, which is set by default and commit every single SQL statement. In an inner join of two tables, each row of the first table is combined joined with every row of second table. This is important, as in some queries, you are interested to have result on every row on the left table, with no match in the right table, e.
Peter runs a small car rental company with 10 cars and 5 trucks. He engages you to design a web portal to put his operation online. A vehicle, identified by the vehicle registration number, can be rented on a daily basis. The rental rate is different for different vehicles.
A customer can rental a vehicle from a start date to an end date. The initial database contains 3 tables: We shall begin with the first table called " products " with the following columns having data types as indicated and rows: The types are: We use fixed-length string for productCode , as we assume that the productCode contains exactly 3 characters.
To insert a row with values on selected columns only, use: Arithmetic Operators You can perform arithmetic operations on numeric fields using arithmetic operators, as tabulated below: NULL cannot be compared.
SET command, with the following syntax: Use with extreme care! Records are NOT recoverable!!! You need to provide the path absolute or relative and the filename. If you use another delimiter, e. For example, to restore the southwind backup earlier: One-To-Many Relationship Suppose that each product has one supplier, and each supplier supplies one or more products.
For example, to list the product's name in products table and supplier's name in suppliers table , we could join the two table via the two common supplierID columns: Many-To-Many Relationship Suppose that a product has many suppliers; and a supplier supplies many products in a so-called many-to-many relationship.
One-to-one Relationship Suppose that some products have optional data e.
You can restore from the backup by running the script either: Foreign Key A foreign key of a child table is used to reference the parent table. We define the foreign key when defining the child table, which references a parent table, as follows: Cannot delete or update a parent row: Indexes or Keys Indexes or Keys can be created on selected column s to facilitate fast search.
In MySQL, indexes can be built on: The valid range is ' You can set a value using the valid format e. The range is '' to ''. YEAR 4 2: The range of years is to However the formatting is all wrong. This column was added to establish the one-to-many relationship.
Putting the supplier's data into the products table results in duplication of data. The primary key of the table consists of two columns: If you use another delimiter, e. As you know, proper indexing requires that the key field contain only unique values.