Editorial Team · on 13 June 2026 · 7 min read · Last reviewed 13 June 2026
SQL (Structured Query Language) is a standardized programming language used to manage and manipulate relational databases, enabling users to insert, update, and delete data efficiently.
Key facts
SQL is pronounced as “ess-que-el” and is the standard language for relational database management systems (RDBMS).
The basic SQL commands for data manipulation are INSERT, UPDATE, and DELETE.
SQL uses tables, which are organized into rows and columns, to store data.
SQL syntax is not case-sensitive, but many developers capitalize SQL keywords to distinguish them from other elements.
SQL is widely used in back-end development, data analysis, and database administration.
How do you insert data into a SQL database?
To insert data into a SQL database, you use the INSERT INTO statement. This command adds new rows of data to a table. The basic syntax is:
Alternatively, you can insert data from another table using the INSERT INTO SELECT syntax:
INSERT INTO NewCustomers (CustomerName, ContactName, Address)
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Country = 'USA';
How do you update existing data in a SQL database?
The UPDATE statement is used to modify existing records in a table. The basic syntax is:
UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;
The WHERE clause is crucial as it specifies which rows to update. Without it, the UPDATE statement will modify all rows in the table.
For instance, to update the address of a customer named ‘Cardinal’, you would use:
UPDATE Customers
SET Address = 'Skagen 22'
WHERE CustomerName = 'Cardinal';
To update multiple columns at once, simply list them with their new values:
UPDATE Customers
SET ContactName = 'Juan J. Lombana', Address = 'Avenida de la Constitución 2341'
WHERE CustomerName = 'Rattlesnake Canyon Grocery';
You can also update data based on values from another table using the UPDATE JOIN syntax:
UPDATE Customers
SET Customers.Address = Orders.ShipAddress
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
WHERE Orders.OrderDate > '2023-01-01';
How do you delete data from a SQL database?
The DELETE statement removes rows from a table. The basic syntax is:
DELETE FROM table_name
WHERE condition;
Again, the WHERE clause is essential to avoid deleting all rows in the table. To delete a specific customer record, you might use:
DELETE FROM Customers
WHERE CustomerName = 'Cardinal';
To delete all records from a table, omit the WHERE clause:
For deleting data based on conditions from another table, use the DELETE JOIN syntax:
DELETE c
FROM Customers c
INNER JOIN Orders o ON c.CustomerID = o.CustomerID
WHERE o.OrderDate < '2020-01-01';
How do you select data from a SQL database?
The SELECT statement is used to retrieve data from a database. The basic syntax is:
SELECT column1, column2, ...
FROM table_name
WHERE condition;
The WHERE clause is optional and is used to filter records. To select all columns from a table, use the asterisk (*) symbol:
SELECT *
FROM Customers;
To select specific columns, list them separated by commas:
SELECT CustomerName, ContactName
FROM Customers;
You can also sort the results using the ORDER BY clause:
SELECT CustomerName, ContactName
FROM Customers
ORDER BY CustomerName;
What are the differences between INSERT, UPDATE, DELETE, and SELECT?
In plain terms: Think of a SQL table like a spreadsheet. INSERT is like adding new rows to the bottom, UPDATE is like editing existing cells, DELETE is like removing entire rows, and SELECT is like viewing specific cells or rows based on your criteria.
Command
Purpose
Syntax
Example
INSERT
Add new data
INSERT INTO table_name (columns) VALUES (values);
INSERT INTO Customers (CustomerName, ContactName, Address) VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21');
UPDATE
Modify existing data
UPDATE table_name SET column = value WHERE condition;
UPDATE Customers SET Address = 'Skagen 22' WHERE CustomerName = 'Cardinal';
DELETE
Remove existing data
DELETE FROM table_name WHERE condition;
DELETE FROM Customers WHERE CustomerName = 'Cardinal';
SELECT
Retrieve data
SELECT columns FROM table_name WHERE condition;
SELECT CustomerName, ContactName FROM Customers WHERE Country = 'USA';
What are some best practices for data manipulation with SQL?
When working with SQL data manipulation, consider the following best practices:
Always use the WHERE clause with UPDATE and DELETE to avoid accidentally modifying or deleting all records.
Use transactions to group multiple operations into a single, atomic unit. This ensures that all operations are completed successfully or none are, preventing partial updates.
Backup your data before performing mass updates or deletes.
Limit the number of columns to update. Only update the columns that need to be changed.
Consider using stored procedures for complex data manipulation tasks. These can be tested and optimized independently.
Use parameterized queries to prevent SQL injection attacks, which can compromise data integrity and security.
How can you ensure data integrity when manipulating data with SQL?
Data integrity refers to the accuracy, consistency, and reliability of data over its lifecycle. To maintain data integrity during manipulation:
First, use constraints such as PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK to enforce rules at the table level.
Second, consider using triggers to automatically perform certain actions when data is inserted, updated, or deleted. For example, you might use a trigger to automatically update a modified date column whenever a record is updated.
Lastly, always validate and sanitize user input to prevent SQL injection attacks, which can compromise data integrity and security.
Constraint
Purpose
Example
PRIMARY KEY
Uniquely identifies each record in a table
ALTER TABLE Customers
ADD CONSTRAINT PK_CustomerID PRIMARY KEY (CustomerID);
ALTER TABLE Customers
ADD CONSTRAINT UQ_CustomerEmail UNIQUE (Email);
NOT NULL
Ensures a column cannot have NULL values
ALTER TABLE Customers
ALTER COLUMN CustomerName SET NOT NULL;
CHECK
Ensures all values in a column satisfy a specific condition
ALTER TABLE Customers
ADD CONSTRAINT CHK_CustomerName CHECK (CustomerName LIKE '[A-Za-z]%');
How can you optimize SQL queries for better performance?
Optimizing SQL queries can significantly improve the performance of your database operations. Here are some strategies to optimize your SQL queries:
First, ensure that your tables are properly indexed. Indexes can greatly speed up data retrieval operations. However, be mindful that indexes also have a cost, as they consume storage space and slow down data manipulation operations.
Second, avoid using SELECT * in your queries. Instead, specify only the columns you need. This reduces the amount of data that needs to be read from the disk and sent over the network.
Third, use the EXPLAIN statement to analyze your queries. This provides information about how the database engine executes your queries, allowing you to identify and fix performance bottlenecks.
Lastly, consider using query hints to provide the database engine with additional information about how to execute your queries. However, use hints sparingly, as they can sometimes lead to unexpected behavior.
Strategy
Description
Example
Indexing
Create indexes on columns that are frequently used in WHERE clauses, JOIN conditions, and ORDER BY clauses.
CREATE INDEX IX_Customers_CustomerName
ON Customers (CustomerName);
Avoid SELECT *
Specify only the columns you need in your SELECT statements.
SELECT CustomerName, ContactName
FROM Customers
WHERE Country = 'USA';
Use EXPLAIN
Analyze your queries to identify performance bottlenecks.
EXPLAIN SELECT CustomerName, ContactName
FROM Customers
WHERE Country = 'USA';
Use Query Hints
Provide the database engine with additional information about how to execute your queries.
SELECT /*+ INDEX(Customers IX_Customers_CustomerName) */
CustomerName, ContactName
FROM Customers
WHERE Country = 'USA';
For more guidance on writing efficient SQL queries, see Best Practices for Writing Efficient SQL Queries. Mastering these commands and practices will enable you to effectively manage and manipulate data in SQL databases, whether you're a developer or a data analyst.
Additionally, consider exploring advanced SQL topics such as window functions, common table expressions (CTEs), and recursive queries to further enhance your SQL skills. These features can help you write more complex and powerful queries to solve real-world data problems.
Always remember to test your queries thoroughly and monitor their performance in a production environment. This will help you ensure that your queries are not only correct but also efficient and scalable.
Frequently asked questions
How do I insert new data into a SQL table?
Use the INSERT INTO statement. Specify the table name and column names, then provide values. Example: INSERT INTO Customers (Name, Email) VALUES ('John Doe', '[email protected]'). Omit column names to insert into all columns, but provide values in table order.
What is the correct syntax for updating records in SQL?
Use the UPDATE statement. Specify the table name, set the column(s) to update with new values, and use a WHERE clause to target specific records. Example: UPDATE Customers SET Email = '[email protected]' WHERE Name = 'John Doe'.
How can I delete data from a SQL table?
Use the DELETE FROM statement. Specify the table name and use a WHERE clause to target specific records. Example: DELETE FROM Customers WHERE Name = 'John Doe'. Always verify your WHERE clause to avoid unintended data loss.
Why is the WHERE clause crucial in UPDATE and DELETE statements?
The WHERE clause filters which records are affected. Without it, all records in the table are updated or deleted. Always include a WHERE clause unless your intent is to modify or remove every record in the table.
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