PostgreSQL（SQL）如果那么那么需要语法

Question

我正在尝试创建以下函数来检查是否存在某些内容。它应该返回true或false。然后我创建了一个触发器来执行该功能。到处都不起作用/语法错误。求救！

CREATE FUNCTION checkForGPU() RETURNS BOOLEAN AS $approved$
    BEGIN
        IF EXISTS (
            SELECT C.ID
            FROM Components C, ComputerComponents CC, ComponentType T, Computer PC
            WHERE CC.ComputerID = C.ID  
            AND CC.ComputerID = PC.ID
            AND C.Type = T.ID
            AND T.Name = 'GPU')
        THEN 
            RETURN TRUE
        ELSE THEN
        RETURN FALSE

    END;    
    $approved$ LANGUAGE plpgsql;


CREATE TRIGGER checkGPU
AFTER INSERT OR UPDATE OR DELETE ON ComputerComponents
EXECUTE PROCEDURE checkForGPU();

Answer 1

如果没有IF-THEN-ELSE语句

，您的功能可以更简单地重写

CREATE FUNCTION checkForGPU() RETURNS BOOLEAN AS $approved$
    BEGIN
        RETURN EXISTS(
            SELECT C.ID
            FROM Components AS C, ComputerComponents AS CC, ComponentType AS T, Computer AS PC
            WHERE CC.ComputerID = C.ID  
            AND CC.ComputerID = PC.ID
            AND C.Type = T.ID
            AND T.Name = 'GPU'
        );
    END;    
    $approved$ LANGUAGE plpgsql;