Come verificare se una variabile è NULL, quindi impostarla con una procedura memorizzata MySQL?

Question

Ho una procedura memorizzata MySQL dove trovo il valore massimo da una tabella.

Se non c'è alcun valore, voglio impostare la variabile sulla data di ieri.

DECLARE current_procedure_name CHAR(60) DEFAULT 'accounts_general'; 
DECLARE last_run_time datetime DEFAULT NULL; 
DECLARE current_run_time datetime DEFAULT NOW(); 

-- Define the last run time 
SET last_run_time := (SELECT MAX(runtime) 
FROM dynamo.runtimes WHERE procedure_name = @current_procedure_name); 

-- if there is no last run time found then use yesterday as starting point 
IF(@last_run_time IS NULL) THEN 
    SET last_run_time := DATE_SUB(NOW(), INTERVAL 1 DAY); 
END IF; 

SELECT @last_run_time; 

Il problema è che @last_run_time è sempre NULL.

Il seguente codice non viene eseguito per qualche motivo

IF(last_run_time IS NULL) THEN 
    SET last_run_time := DATE_SUB(NOW(), INTERVAL 1 DAY); 
END IF; 

Come posso impostare la variabile @last_run_time correttamente?

Answer 1

@last_run_time è un 9.4. User-Defined Variables e last_run_time datetime uno 13.6.4.1. Local Variable DECLARE Syntax, sono variabili diverse.

Prova: SELECT last_run_time;

UPDATE

Esempio:

/* CODE FOR DEMONSTRATION PURPOSES */ 
DELIMITER $$ 

CREATE PROCEDURE `sp_test`() 
BEGIN 
    DECLARE current_procedure_name CHAR(60) DEFAULT 'accounts_general'; 
    DECLARE last_run_time DATETIME DEFAULT NULL; 
    DECLARE current_run_time DATETIME DEFAULT NOW(); 

    -- Define the last run time 
    SET last_run_time := (SELECT MAX(runtime) FROM dynamo.runtimes WHERE procedure_name = current_procedure_name); 

    -- if there is no last run time found then use yesterday as starting point 
    IF(last_run_time IS NULL) THEN 
     SET last_run_time := DATE_SUB(NOW(), INTERVAL 1 DAY); 
    END IF; 

    SELECT last_run_time; 

    -- Insert variables in table2 
    INSERT INTO table2 (col0, col1, col2) VALUES (current_procedure_name, last_run_time, current_run_time); 
END$$ 

DELIMITER ;