Why Am I Getting an \'Undeclared Variable\' Error with SELECT INTO in MySQL?

SELECT INTO and the "Undeclared Variable" Error

When attempting to execute a query that uses the SELECT INTO syntax to insert data into a variable as follows:

SELECT id_subscriber
  INTO newsletter_to_send
  FROM subscribers 

An error "1327 - Undeclared variable: newsletter_to_send" might be encountered. This error occurs because MySQL does not support SELECT INTO for variables.

Solution

To insert data into a table using the result of a query, the INSERT ... SELECT syntax should be used instead:

INSERT INTO newsletter_to_send
SELECT id_subscriber FROM subscribers 

This query will insert the id_subscriber values from the subscribers table into the newsletter_to_send table.

Additional Considerations

When using INSERT ... SELECT, consider including a WHERE clause to filter the data being inserted. Additionally, ensure that the table specified in the INTO clause already exists before executing the query.

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