SQL SELECT and SELECT WHERE (With Examples)

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SQL SELECT and SELECT WHERE (With Examples)

2023-12-03 04:55| 来源: 网络整理| 查看: 265

The SQL SELECT statement is used to select (retrieve) data from a database table.

Example -- select first_name from Customers table SELECT first_name FROM Customers;

The above SQL query selects the first_name of all the customers from the Customers table.

SQL SELECT Syntax

The syntax of the SQL SELECT statement is:

SELECT column1, column2, ... FROM table;

Here,

column1, column2, ... are the table columns table is the table name from where we select the data

For example,

-- select first_name and last_name columns from Customers table SELECT first_name, last_name FROM Customers;

Here, the SQL command selects the first_name and last_name of all customers in the Customers table.

Example: SQL SELECT SQL SELECT ALL

To select all columns from a database table, we use the * character. For example,

-- select all columns from Customers table SELECT * FROM Customers;

Here, the SQL command selects all columns of the Customers table.

Example: SQL SELECT All SQL SELECT WHERE Clause

A SELECT statement can have an optional WHERE clause. The WHERE clause allows us to fetch records from a database table that matches specified condition(s). For example,

-- select all columns from the customers table with last_name 'Doe' SELECT * FROM Customers WHERE last_name = 'Doe';

Here, the SQL command selects all customers from the Customers table with the last_name Doe.

Example: SQL SELECT with WHERE

Let's look at another example.

-- select age and country columns from customers table where the country is 'USA' SELECT age, country FROM Customers WHERE country = 'USA';

Here, the SQL command selects the age and country columns of all the customers whose country is USA.

Example: SQL SELECT with WHERE

We can also use the WHERE clause with the UPDATE statement to edit existing rows in a database table.

Note: In SQL, we must enclose textual data inside either single or double quotations like 'USA'.

SQL Operators

The WHERE clause uses operators to construct conditions. Some of the commonly used operators are:

1. Equal to Operator (=)

-- select all columns from Customers table with first name 'John' SELECT * FROM Customers WHERE first_name = 'John';

The above SQL command selects all the customers from the Customers table having first_name John.

2. Greater than (>)

-- select all columns from Customers table with age greater than 25 SELECT * FROM Customers WHERE age > 25;

The above SQL command selects all the customers from the Customers table whose age is greater than 25.

3. AND Operator (AND)

-- select all columns from Customers table with last_name 'Doe' and country 'USA' SELECT * FROM Customers WHERE last_name = 'Doe' AND country = 'USA';

The above SQL command selects all the customers from the Customers table having last_name Doe and country USA.

Note: If none of the rows meet the WHERE clause condition, an empty result set is returned.

To learn more about all the SQL operators in detail, visit SQL Operators.

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SQL SELECT and SELECT WHERE (With Examples)

SQL SELECT and SELECT WHERE (With Examples)

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