Discuss the basic knowledge and conversion techniques of Oracle SQL

In database management and development, SQL is a query language that must be mastered. Among the SQL languages, Oracle SQL is the most important and widely used dialect. In this article, we will discuss the basic knowledge and conversion skills of Oracle SQL to help everyone become more proficient in using this kind of SQL.

Basic Syntax

Like other SQL dialects, in Oracle SQL, users query data in the database by using the SELECT statement. The following is a basic query syntax:

SELECT column1, column2, …
FROM table_name
WHERE condition;

In the SELECT statement, column1, column2, …represent The columns that need to be displayed in the query results, table_name represents the table from which the data comes, and condition represents the conditions of the query.

For example, to query the names of all people living in "New York City" in a table called customers, we can use the following query statement:

SELECT name
FROM customers
WHERE city = 'New York City';

This query statement will return a result set containing names, which belong to customers who live in "New York City".

Another important statement is INSERT, which allows inserting new data into the table. The following is a syntax for inserting data:

INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);

In this statement , table_name is the name of the table where data is to be inserted, column1, column2, ... represent the columns that need to be filled in the table, value1, value2, ... are the data values ​​of the corresponding columns.

For example, if we want to insert a new order into a table called orders, including customer ID, order date, product name, quantity and price, we can use the following statement:

INSERT INTO orders (customer_id, order_date, product_name, quantity, price)
VALUES (1001, '2021-01-05', 'Widget', 10, 9.99);

This INSERT statement will send orders to Insert a new order record into the table.

Data type conversion

When crossing different databases or connecting different applications, we sometimes need to perform data type conversion. In Oracle SQL, several functions are provided to handle this situation.

1. TO_CHAR

TO_CHAR function is used to convert different types of data into string types.

SELECT TO_CHAR(1234.56, '9999.99') as num FROM dual;

The output result is: 1,234.56

2. TO_NUMBER

TO_NUMBER converts a string to a numeric type. If the string cannot be converted to a number, a null value or an error message will be returned.

SELECT TO_NUMBER('1234', '9999') as num FROM dual;

The output result is: 1234

3. TO_DATE

TO_DATE function is used to convert string to date type.

SELECT TO_DATE('01-JAN-2021', 'DD-MON-YYYY') as order_date FROM dual;

The output result is: 2021-01-01

4. CAST

The CAST function converts one data type to another data type.

SELECT CAST('1234.56' as float) as num FROM dual;

The output result is: 1234.56

Handling of null values

In Oracle database , null values ​​are represented by NULL. The NULL value is not equal to zero because it represents an unknown value, which makes restoring the data very difficult.

When using query statements, we often need to determine the situation of null values. In order to determine whether it is a null value, there are two ways to use:

1. IS NULL

Use IS NULL in the WHERE clause to determine whether a column contains null values. . For example:

SELECT name, email
FROM customers
WHERE email IS NULL;

In this query, we will return the names and emails of all customers whose email is NULL address.

2. NVL

The NVL function allows replacing null values ​​with specified values. For example, if we want to replace the null values ​​in the prices table with 0, we can use the following statement:

SELECT NVL(price, 0) as price
FROM prices;

This query statement will Returns a result set containing all price values. Records with null values ​​will be replaced with 0.

Summary

In Oracle database management and development, it is very important to be proficient in SQL queries and basic skills. This article introduces the basic syntax and conversion techniques of Oracle SQL, including data type conversion and null value processing. Of course, Oracle SQL is very rich in functions. What is introduced here is just some basic knowledge. I hope it can help everyone better understand and use Oracle SQL.

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