How to set and use variable in a trigger?

I’m creating a trigger in MySQL that should set a variable and use it to update a table. But i can’t find the right syntax to make this work.

This is what i tried:

CREATE TRIGGER after_assinatura_update AFTER UPDATE
ON documento_eletronico_assinatura
FOR EACH ROW
BEGIN
    SET @prox_ass = (
        SELECT dea.usuario_id
        FROM documento_eletronico_assinatura dea
        INNER JOIN usuario u ON dea.usuario_id = u.id
        WHERE dea.documento_id = NEW.documento_id
        AND dea.assinatura = 0
        ORDER BY dea.posicao ASC
        LIMIT 1
    );/*ERROR: statement incomplete, expecting: ';'*/

    UPDATE documento_eletronico 
    SET documento_eletronico.prox_assinatura = @prox_ass
    WHERE documento_eletronico.id = NEW.documento_id;

CREATE TRIGGER after_assinatura_update AFTER UPDATE
ON documento_eletronico_assinatura
FOR EACH ROW
    UPDATE documento_eletronico 
    SET documento_eletronico.prox_assinatura = @prox_ass
    WHERE documento_eletronico.id = NEW.documento_id;
/*TRIGGER stops here and doesn't set the variable. Adding a BEGIN gives the "expected: ';'" at The line above.*/
    SET @prox_ass = (
        SELECT dea.usuario_id
        FROM documento_eletronico_assinatura dea
        INNER JOIN usuario u ON dea.usuario_id = u.id
        WHERE dea.documento_id = NEW.documento_id
        AND dea.assinatura = 0
        ORDER BY dea.posicao ASC
        LIMIT 1
    );

Answer

You probably just need a DELIMITER statement. However, you can express this as one statement:

SET @prox_ass = (
    /*ERROR: statement incomplete, expecting: ';'*/

UPDATE documento_eletronico de CROSS JOIN
       (SELECT dea.usuario_id
        FROM documento_eletronico_assinatura dea JOIN
             usuario u
             ON dea.usuario_id = u.id
        WHERE dea.documento_id = NEW.documento_id AND
              dea.assinatura = 0
        ORDER BY dea.posicao ASC
        LIMIT 1
       ) u
);
SET de.prox_assinatura = u.usuario_id
WHERE de.id = NEW.documento_id;