How to query a table in oracle

Oracle database is the most commonly used relational database, and its query language is SQL. This article will briefly introduce how to use SQL query statements to query tables in Oracle database.

1. Prerequisites

Before you can query the Oracle database, you need to have the following:

1. Oracle database account
2. can be connected to Oracle database client
3. Understand the basic knowledge of Oracle SQL query language

2. Use the SELECT query statement to query the table

1. Query all the items in the table Column

In Oracle database, the most basic SELECT statement is used to query all columns in the table.

For example, suppose we have a table named "employees", which includes the fields (columns) "employee_id", "first_name", "last_name", "email", "phone_number", "hire_date" and "job_id". If you want to query all columns in the table, you can use the "*" symbol in the SQL statement.

SELECT * FROM employees;

2. Query specific columns in the table

If you only want to query specific columns in the table, rather than all columns, you can list the columns to be queried in the SELECT statement Column name.

SELECT first_name, last_name, email FROM employees;

3. Query the unique value in the table

Sometimes you may need to query the unique value in a table. This can be achieved by using the DISTINCT keyword in the SELECT statement.

For example, assuming you want to query the unique position ID (job_id), you can use the following SELECT statement:

SELECT DISTINCT job_id FROM employees;

4. Add filter conditions

SELECT Statements can also use a WHERE clause to filter the returned results. The WHERE clause can use comparison operators (such as >, <, =, etc.) and logical operators (such as AND, OR, NOT, etc.) to define filter conditions.

For example, the following SELECT statement will obtain the names and salaries of all employees whose salary is less than $5000.

SELECT first_name, last_name, salary
FROM employees
WHERE salary < 5000;

5. Sort query results

You can also sort the returned results by using the ORDER BY clause in the SELECT statement.

For example, the following SELECT statement will sort employees by hire_date (hire date) in ascending order.

SELECT employee_id, first_name, last_name, hire_date
FROM employees
ORDER BY hire_date ASC;

6. Using aggregate functions

SQL also provides a series of aggregate functions, such as SUM, AVG, COUNT, MIN and MAX. These aggregate functions can be used to summarize and count column values. Aggregate functions can be used in SELECT statements.

For example, the following SELECT statement will return the average salary of all employees.

SELECT AVG(salary) FROM employees;

3. Summary

This article briefly introduces how to use the SELECT query statement to query tables in the Oracle database. You can query specific columns or all columns by using a basic SELECT statement and add additional filtering and sorting using WHERE clauses, ORDER BY clauses, and various aggregate functions. As long as you master these basic concepts, you can perform efficient data queries in Oracle database.

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