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>

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• 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>

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• ## 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>

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Example

FROM DUAL :

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

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.

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