by Dinesh Thakur Category: SQL Tutorials

The SQL DISTINCT query is used with the SELECT keyword retrieves only unique data values depending on the fields you have specified after it. To illustrate it we use emp table as shown below.

Select ename from EMP;
 
by Dinesh Thakur Category: SQL Tutorials

The DELETE statement is used to delete rows in a table. To do so, we can use the DELETE FROM command. An SQL DELETE statement removes one or more records from a table. A subset may be defined for deletion using a condition, otherwise all records are removed.

 
by Dinesh Thakur Category: SQL Tutorials

Oracle uses work areas to execute SQL statements and store processing information. A PL/SQL construct called a CURSOR lets you name a work area and access its stored information. A Cursor in its simplest form can be thought of as a pointer to the records in database table or a virtual table represented by the result of a SELECT statement.



 
by Dinesh Thakur Category: SQL Tutorials

The create table statement is used to create a new table. The CREATE TABLE statement defines a table. The definition must include its name and the names and attributes of its columns. The definition can include other attributes of the table, such as its primary key or check constraints. This statement comes under the DDL statement.

 
by Dinesh Thakur Category: SQL Tutorials

A sequence is a database object that generates numbers in sequential order. Applications most often use these numbers when they require a unique value in a table such as primary key values. Some database management systems use an "auto number" concept or "auto increment" setting on numeric column types. Both the auto numbering columns and sequences provide a unique number in sequence used for a unique identifier.



 
by Dinesh Thakur Category: SQL Tutorials

SQL (Standard Query Language) is a language for manipulating databases developed in the 70s by IBM. All data management systems use SQL to access data or to communicate with a data server. RDBMS is the core platform for SQL, and for all other modern database languages such as  Oracle, MS SQL Server, IBM DB2, MySQL, and Microsoft Access, PostgreSQL, SQLite, Firebird, and many more. SQL (Standard Query Language) is born as a result of the mathematical work of Codd, who founded the work of relational databases, three types of manipulations on the database:



 
by Dinesh Thakur Category: SQL Queries

Sometimes we may decide that we need to get rid of a table in the database for some reason. The SQL DROP TABLE statement is the SQL command that removes an entire SQL table. This command also removes one or more table definitions and all data, indexes, triggers, constraints, and permission specifications for those tables.



 
by Dinesh Thakur Category: SQL Queries

You can rename a table or a column temporarily by giving another name known as alias. The term alias means an alternate name. Here’s an example of how to use a column alias

 
by Dinesh Thakur Category: SQL Queries

After the table( s) are created and begin to be used, requirements are likely to occur which requires modifications in the structure of the table, such as adding new columns to the table, modifying and existing column's definition etc. So these modifications can be performed using the ALTER table DDL statement. Tills statement changes the structure of the table and not its contents. There are many types of modifications that can be made to the structure of the tables. Using ALTER TABLE statement, you can make modifications such as

 
by Dinesh Thakur Category: SQL Queries

The SQL BETWEEN & AND operator selects a range of data between two values. You can also use the BETWEEN function with dates. The BETWEEN function can also be combined with the NOT operator

 
by Dinesh Thakur Category: SQL Queries

Check constraint is used to validate values entered into a column. CHECK constraints enforce domain integrity by limiting the values that are accepted by a column. They are similar to FOREIGN KEY constraints in that they control the values that are placed in a column. The difference is in how they determine which values are valid: FOREIGN KEY constraints get the list of valid values from another table.

 
by Dinesh Thakur Category: SQL Queries

Constraints are used to limit the type of data that can go into a table. Integrity constraints are used to ensure accuracy and consistency of data in a relational database. Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).



 
by Dinesh Thakur Category: SQL Queries

The most difficult part when working with dates is to be sure that the format of the date you are trying to insert, matches the format of the date column in the database. As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated.



 
by Dinesh Thakur Category: SQL Queries

The DEFAULT constraint is used to insert a default value into a column. The default value will be added to all new records, if no other value is specified. You can also specify the DEFAULT value for columns i.e. when user does not enter anything in that column then that column will have the default value. Default constraints are ANSI -standard constraints you can assign to a column either with the CREATE TABLE or ALTER TABLE statements.

 
by Dinesh Thakur Category: SQL Queries

The EXISTS condition is considered "to be met" if the subquery returns at least one row. EXISTS operator simply tests whether the inner query returns any row. If it does, then the outer query proceeds. If not, the outer query does not execute, and the entire SQL statement returns nothing.

 
by Dinesh Thakur Category: SQL Queries

A FOREIGN KEY in one table points to a PRIMARY KEY in another table. On whichever column you put FOREIGN KEY constraint then the values in that column must refer to existing values in the other table.

 
by Dinesh Thakur Category: SQL Queries

The HAVING Clause was added to SQL because the WHERE keyword could not be used with aggregate functions. As we know that WHERE clause is used to restrict the number of rows fetched from the table. But if we try to restrict the number of groups with where clause it will generate an error message. So we can use having clause to divide the groups with Group by clause.

 
by Dinesh Thakur Category: SQL Queries

The IN operator allows you to specify multiple values in a WHERE clause. The IN function helps reduce the need to use multiple OR conditions. The SQL IN clause allows you to specify discrete values in your SQL WHERE search criteria. The syntax for using the IN keyword is as follows:

 
by Dinesh Thakur Category: SQL Queries

Similar to the UNION command, INTERSECT also operates on two SQL statements. The difference is that, while UNION essentially acts as an OR operator (value is selected if it appears in either the first or the second statement), the INTERSECT command acts as an AND operator (value is selected only if it appears in both statements).

 
by Dinesh Thakur Category: SQL Queries

Null is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database. NULL means missing information and inapplicable information". NULL values represent missing unknown data. By default, a table column can hold NULL values.



 

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About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.