DELETE can remove all rows from a table (using ), or can be used as part of a WHERE clause to delete rows that meet a specific condition. The terms int and varchar(255) in this example specify the datatypes of the columns we’re creating. This statement goes through all the conditions and returns a value when the first condition is met. So, if no conditions are TRUE, it returns the value in the ELSE clause.
The OFFSET statement works with ORDER BY and specifies the number of rows to skip before starting to return rows from the query. DROP DATABASE deletes the entire database including all of its tables, indexes etc as well as all the data within it. CREATE can be used to set up a database, table, index or view.
SELECT
The query above sets the isolation level to READ COMMITTED for the transaction. The changes made in the transaction will be visible to other transactions only after they are committed. In the query above, the COMMIT statement is used to permanently save the changes made to the employees and products tables during the transaction. The query above uses the AVG function to calculate the average value of the price column in the products table.
SQL syntax is a unique set of rules and guidelines to be followed while writing SQL statements. This tutorial gives you a quick start with SQL by basic database queries listing all the basic SQL Syntax. TRUNCATE TABLE removes all data entries from a table in a database, but keeps the table and structure in place.
SQL Database
The query above uses the NULLIF function to check if total_amount is equal to discounted_amount. If they are equal, it returns NULL; otherwise, it returns total_amount. The result is returned with the diff_amount indicating the difference (or NULL) between the total amount and the discounted amount for each order.
SUM() is a function that takes the name of a column as an argument and returns the sum of all the values in that column. ORDER BY is a clause that indicates you want to sort the result set by a particular column either alphabetically or numerically. LIMIT is a clause that lets you specify the maximum number of rows the result set will have. An inner join will combine rows from different tables if the join condition is true.
Different Types of SQL Commands
This constraint consists of a set of default values for a column when no value is specified. As the name suggests, this statement is used to create a database. The first one commits a transaction, which basically means that all changes are permanently saved.
In this article on SQL Commands, I will discuss the top commands and statements that you need to understand in SQL. Databases in SQL are a set of interrelated records stored in tables, which, in turn, are divided into columns and rows. The former describe the stored data, and the latter store it. WHERE is a clause that indicates you want to filter the result set to include only rows where the following condition is true. OR is an operator that filters the result set to only include rows where either condition is true.
Data Definition Language (DDL)
In the era where data is being generated in humongous amounts, there is a constant need to handle data in databases. Relational databases are one of the most popular databases, and SQL is the basis of relational databases. Therefore SQL skills are indispensable in most of the job roles.
The below example would give SELECT and UPDATE access on the customers table to a user named “usr_bob”. The query above uses the INTERSECT operator to find the common first_name and last_name between the customers and employees tables. The result will include rows where both the first name and last name are present in both tables. The UNION operator combines the result sets of two or more SELECT statements into a single result set. The query above uses the CHAR_LENGTH function to calculate the length of the product_name column in the products table. DDL or Data Definition Language actually consists of the SQL commands that can be used to define the database schema.
BACKUP DATABASE
It allows you to specify the values to be inserted for each column or use a subquery to retrieve data from another table. Both conditions must be true for the row to be included in the result set. A RIGHT JOIN returns all rows from the right table, and the matched rows from the left table.
The query above uses the COALESCE function to select the first non-null name (either first_name or middle_name) as the preferred_name for each employee. The result is returned with the calculated preferred_name for each employee. The query above uses the IF function to categorize employees based on their age. If the age is greater than 50, they are categorized as Senior; otherwise, they are categorized as Junior. The result is returned with the name, age, and the calculated employee_category. The query above uses the COUNT function to count the non-null values in the age column of the employees table.
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The query above creates a table named employees that contains four columns which are employee_id, first_name, last_name, and age. Data definition language (DDL) is a collection of SQL commands that are used to build, change, and delete database structures. They describe the database schema and are used to design the layout of the objects stored in the database. SQL commands are the foundation of an effective database management system. Whether you are manipulating data, or managing data, SQL provides all sets of tools.
- Data Definition Language (DDL) commands are SQL statements used to create, alter, and drop database objects like tables, indexes, views, and schemas.
- You can use the ALTER TABLE statement with ADD/DROP Column command according to your need.
- The result is returned as a new column named left_product_name.
- These commands allow users to perform various actions on a database.
- Think of it as the “OR” JOIN compared with the “AND” JOIN (INNER JOIN).