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

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 SQL GROUP BY clause is used along with the SQL aggregate functions and specifies the groups where selected rows are placed. When one or more aggregate functions are presented in the SQL SELECT column list, the SQL GROUP BY clause calculates a summary value for each group.

 
by Dinesh Thakur Category: SQL Tutorials

The SQL WHERE Clause is used when you want to retrieve specific information from a table excluding other irrelevant data. In a where clause simple conditions based on comparison operators can be combined using the logical connectives and, or, and not to form complex conditions. Conditions may also include pattern matching operations and even subqueries.



 
by Dinesh Thakur Category: SQL Tutorials

The SQL SELECT INTO statement can be used to retrieves data from a table and inserts it to another database table. The SQL SELECT INTO statement used to create backup copies of tables.



 
by Dinesh Thakur Category: SQL Tutorials

The INSERT statement is used to add new row to a table. To insert new row(s) into a table, the table must be in your own schema or you must have INSERT privilege on the table. Only one row is inserted at a time with this syntax. You can insert literal values (26.5, 'DINESH), expres­sions containing operators and functions, null values etc. only.

 
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

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

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

Use the LIKE condition to perform wildcard searches of valid search string values. Search conditions can contain either literal characters or numbers. You can combine pattern-matching characters. You can use the ESCAPE identifier to search for the actual % and _ symbols.

 
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

The PRIMARY KEY constraint uniquely identifies each record in a database table. Primary keys must contain unique values. A primary key column cannot contain NULL values. Each table should have a primary key, and each table can have only one primary key.



 
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

By default all columns in a table can contain null values. If you want to ensure that a column must always have a value, i.e. it should not be left blank, then define a NOT NULL constraint on it.

 
by Dinesh Thakur Category: SQL Queries

A UNIQUE key integrity constraint requires that every value in a column or set of columns (key) be unique-that is, no two rows of a table have duplicate values in a specified column or set of columns.

 
by Dinesh Thakur Category: SQL Queries

In SQL the UNION Clause combines the results of two SQL queries into a single table of all matching rows. The two queries must have the same number of columns and compatible data types to unite. Any duplicate records are automatically removed unless UNION ALL is used.



 
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

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

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.



 

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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.