How to query a table in oracle

Oracle Query a Table

In the Oracle database, the table in the database is one of the most basic storage objects, and all data is stored and operated in the table. In daily database management and data processing, it is often necessary to query the data in a certain table. The following will introduce in detail the methods and steps of Oracle querying a certain table.

1. Use the select statement to query the table

Using the select statement is the most common method of querying the table. Its syntax format is:

SELECT column1, column2, …
FROM table_name
WHERE condition;

Among them, column1, column2, … is the name of the data column that needs to be queried, you can use * to represent querying all columns; table_name is the name of the table that needs to be queried; condition is the query condition, which can be omitted. For example, to query all data in the table named employee, you can use the following statement:

SELECT * FROM employee;

If you only need to query certain columns in the table, you can specify the column names to be queried, for example:

SELECT employee_name, employee_salary FROM employee;

If you need to set up a conditional query, you can specify the query conditions in the WHERE clause. For example, to query employee information with a salary greater than 5,000, you can use the following statement:

SELECT * FROM employee WHERE employee_salary > 5000;

2. Use the desc command to query the table structure

In the Oracle database, you can use the desc command to query the structure and field information of the table. The syntax format is:

DESC table_name;

For example, to query the structure of a table named employee, you can use the following statement:

DESC employee;

After execution, all field information of the table will be returned, including field names and data types. , length, etc.

3. Use the show command to query the table creation statement

In the Oracle database, you can use the show command to query the table creation statement. The syntax format is:

SHOW CREATE TABLE table_name;

For example, to query the creation statement of a table named employee, you can use the following statement:

SHOW CREATE TABLE employee;

After execution, the SQL statement to create the table will be returned, including the table name, Field name, data type, length, default value, primary key and other information.

4. Use the data dictionary to query table information

In the Oracle database, you can also query table-related information through the system data dictionary table. The system data dictionary is some metadata information stored internally in the Oracle system and can be queried through the views provided by the system. Commonly used data dictionary views include:

• ALL_TABLES: Contains table information accessible to all users;
• ALL_TAB_COLUMNS: Contains column information of tables accessible to all users;
• ALL_CONSTRAINTS: Contains the constraint information of tables accessible to all users;
• USER_TABLES: Contains the table information owned by the current user;
• USER_TAB_COLUMNS: Contains the table information owned by the current user Column information of the table;
• USER_CONSTRAINTS: Contains the constraint information of the table owned by the current user.

For example, to query the names and data types of all columns in the table named employee, you can use the following statement:

SELECT column_name, data_type FROM all_tab_columns WHERE table_name = 'employee';

After execution, the names and data types of all columns in the table will be returned.

Summary:

The above introduces the methods and steps for querying a certain table in Oracle, including using the select statement to query the table, using the desc command to query the table structure, using the show command to query the table creation statement and using Data dictionary query table information. Different methods can be selected and used according to different needs and purposes. I hope this article can help everyone in daily database management and data processing.

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