Insert, Update, Delete Query in SQL

 

What is Query

In SQL, a query is a statement that retrieves or manipulates data from a database. A query typically starts with the SELECT keyword, followed by a list of columns that should be returned in the result set. It may also include additional clauses such as FROM, WHERE, GROUP BY, HAVING, and ORDER BY to filter, group, and sort the data.

For example, the following SQL query retrieves all the rows from a table called "customers" that have a "city" column value of "New York":

This query selects all columns from the "customers" table and applies a filter to only include rows where the "city" column value is "New York". The result set would include all the columns for each matching row in the "customers" table.

Insert Query:

In SQL, the INSERT statement is used to add new rows of data to a table. The basic syntax of an INSERT statement is as follows:

The table_name is the name of the table where you want to insert data, and the column1, column2, column3, etc. represent the names of the columns that you want to insert data into. The VALUES keyword is followed by a list of values that correspond to the columns specified in the INSERT INTO clause.

For example, suppose you have a table called customers with columns id, name, email, and phone. You can use the following INSERT statement to add a new row of data to the table:

This statement would insert a new row into the customers table with the name "John Doe", email "johndoe@email.com", and phone number "555-1234". The id column is assumed to be an auto-incrementing primary key and is not specified in the INSERT statement.

Note that if you omit a column from the column list in the INSERT statement, you must provide a value for that column using a default value or a NULL value. If a column is not included in the INSERT statement and has no default value, the INSERT statement will fail.

Update Query:

In SQL, the UPDATE statement is used to modify existing rows of data in a table. The basic syntax of an UPDATE statement is as follows:

The table_name is the name of the table that you want to update, and the column1, column2, etc. represent the names of the columns that you want to update. The value1, value2, etc. represent the new values that you want to set for the corresponding columns. The WHERE clause specifies which rows to update based on a certain condition.

For example, suppose you have a table called customers with columns id, name, email, and phone. You can use the following UPDATE statement to modify the email and phone number of a customer with ID 1:

This statement would update the email and phone number of the customer with ID 1 in the customers table.

Note that if you omit the WHERE clause, the UPDATE statement will modify all rows in the table. Therefore, it is important to include a condition in the WHERE clause to ensure that only the desired rows are updated.

Delete Query:

In SQL, the DELETE statement is used to remove one or more rows from a table. The basic syntax of a DELETE statement is as follows:

The table_name is the name of the table from which you want to delete rows. The WHERE clause specifies which rows to delete based on a certain condition.

For example, suppose you have a table called customers with columns id, name, email, and phone. You can use the following DELETE statement to remove a customer with ID 1 from the table:

This statement would delete the row with ID 1 from the customers table.

Note that if you omit the WHERE clause, the DELETE statement will remove all rows from the table. Therefore, it is important to include a condition in the WHERE clause to ensure that only the desired rows are deleted.

It is also worth noting that when you delete a row from a table, any related rows in other tables may also be affected depending on the database's referential integrity rules. For example, if the customers table has a foreign key constraint that references a orders table, deleting a customer may also delete any related orders.