What does from dual mean in sql

In SQL, the FROM DUAL statement retrieves one row and one column of data from a special virtual table DUAL that contains only one row with the value X. Common uses include: initialization sequences, inserting default values, and as a virtual table in subqueries.

The meaning of FROM DUAL in SQL

In SQL, the FROM DUAL statement is used To retrieve a row of data from a special virtual table called DUAL. This table has only one row and one column, the column names are always DUMMY, and the values ​​are always X.

Usage scenarios

The following are some common scenarios for using FROM DUAL:

• Initialization sequence: You can use FROM DUAL to initialize the sequence, for example:

  <code class="sql">CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;
  ALTER SEQUENCE my_sequence OWNED BY my_table.my_column;</code>

• Insert default value: You can use FROM DUAL Set default values ​​for columns in the table, for example:

  <code class="sql">ALTER TABLE my_table ADD COLUMN my_column INT DEFAULT (SELECT 1 FROM DUAL);</code>

• ## Subquery: FROM DUAL can be used as A virtual table in a subquery, for example:

  <code class="sql">SELECT * FROM (SELECT 1 FROM DUAL) AS subquery;</code>

Example

Here is a simple example using

FROM DUAL :

<code class="sql">SELECT 1 FROM DUAL;</code>

This query returns one row of data, the row contains an integer value 1.

Note:

The DUAL table is a read-only table and data cannot be inserted, updated, or deleted in it.
• FROM DUAL is for convenience only, it does not perform any actual operation. It essentially just returns a virtual table containing one row of data.

The above is the detailed content of What does from dual mean in sql. For more information, please follow other related articles on the PHP Chinese website!