sql - 如何在存储过程中检查两个 SELECT 语句的输出是否等于、大于或小于？

Question

我制作了一个数据库来存储类(class)学生和教授，以便制作注册系统。但是，我试图创建一个存储过程来检查“Sections”表中的“classSize”属性是否大于或等于“classSizeCap”属性。但是，我遇到了问题。 PostgreSQL 无法识别我的 CASE 或 IF 语句。我该如何解决这个问题？

    -- WILL SHOW WHAT STUDENTS NEED OVERRIDES FOR WHICH CLASS --
    CREATE OR REPLACE FUNCTION startOverrides(REFCURSOR) RETURNS refcursor AS
    $$
    DECLARE
        class_size    INT        := (PERFORM Sections.classSize 
                                     FROM Sections 
                                     INNER JOIN Enrollment ON Enrollment.courseID = Sections.courseID 
                                     WHERE Enrollment.sectionNumber = Sections.sectionNumber);
        class_members INT        := (PERFORM students_enrolled.courseName, COUNT(*) 
                                    FROM students_enrolled
                                    GROUP BY students_enrolled.courseName, COUNT(*));
        overrides     REFCURSOR  := $1;
    BEGIN
        OPEN overrides FOR
            CASE WHEN  class_size >= class_members 
            THEN SELECT students_enrolled.fname, students_enrolled.lname, students_enrolled.courseName, SUM(Sections.classSizeCap - Sections.classSize) AS overrides
                 FROM students_enrolled
                 INNER JOIN Sections ON Sections.courseID = students_enrolled.courseID
                                     AND Sections.sectionNumber = students_enrolled.sectionNumber
                 GROUP BY students_enrolled.fname, students_enrolled.lname, students_enrolled.courseName, SUM(Sections.classSizeCap - Sections.classSize)
            END IF;
        RETURN overrids;
    END
    $$ 
    LANGUAGE plpgsql;
    SELECT students_enrolled('overrides');
    FETCH ALL FROM overrides;

我的错误:

ERROR:  syntax error at or near "CASE"
LINE 8:   CASE WHEN (SELECT Sections.classSize 
          ^
********** Error **********

ERROR: syntax error at or near "CASE"
SQL state: 42601
Character: 151

这是我主要使用的 View :

    CREATE OR REPLACE VIEW students_enrolled AS
        SELECT People.fname, People.lname, Courses.courseID, Courses.courseName, Students.creditsEarned
        FROM Enrollment 
        INNER JOIN People ON Enrollment.studentID = People.pid
        INNER JOIN Students ON Enrollment.studentID = Students.studentID
        INNER JOIN Courses ON Enrollment.courseID = Courses.courseID
        GROUP BY People.fname, People.lname, Courses.courseID, Courses.courseName, Students.creditsEarned;

这是我的数据库的快照:

    
    -- HOLDS A SPECIFIC COURSE WITHOUT THE INSTANCES OF THE CLASS --
    CREATE TABLE Courses (
        courseID      SERIAL      UNIQUE NOT NULL,
        department    TEXT               NOT NULL,
        courseNumber  VARCHAR(10)        NOT NULL,
        courseName    TEXT        UNIQUE NOT NULL,
        credits       INT                NOT NULL,
        PRIMARY KEY(courseID)
    );
    
    -- HOLDS A SPECIFIC COURSE OFFERINGS --
    CREATE TABLE ClassesAvailable (
        courseID INT  NOT NULL,
        year     INT  NOT NULL,
        term     TEXT NOT NULL,
        CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
        PRIMARY KEY(courseID, year, term)
    );
    
    -- PEOPLE SUPERTYPE --
    CREATE TABLE People (
        pid   SERIAL            UNIQUE NOT NULL,
        fname TEXT                     NOT NULL,
        lname TEXT                     NOT NULL,
        PRIMARY KEY(pid)
    );
    
    -- HOLDS THE DIFFERENT PROFESSORS TEACHING AT THE SCHOOL --
    -- SUBTYPE OF PEOPLE --
    CREATE TABLE Professors (
        professorID  INT  UNIQUE NOT NULL,
        status       TEXT        NOT NULL,
        CHECK(status = 'Full-Time' OR status = 'Part-time'),
        PRIMARY KEY(professorID),
        FOREIGN KEY(professorID) REFERENCES People(pid)
    );
    
    -- HOLDS THE SPECIFIC INSTANCES OF THE CLASS DEPENDING ON THE YEAR AND TERM --
    CREATE TABLE Sections (
        courseID      INT          NOT NULL,
        year          INT          NOT NULL,
        term          TEXT         NOT NULL, 
        sectionNumber INT          NOT NULL,
        startDate     DATE         NOT NULL,
        endDate       DATE         NOT NULL,
        crn           INT          NOT NULL,
        classSize     INT          NOT NULL,
        classSizecap  INT          NOT NULL,
        CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
        PRIMARY KEY(courseID, year, term, sectionNumber),
        FOREIGN KEY(courseID, year, term) REFERENCES ClassesAvailable(courseID, year, term)
    );
    
    -- HOLDS THE EVENT OF THE CLASS --
    -- A CLASS MAY HAVE DIFFERENT DAYS ON WHICH --
    -- THEY MEET ON, SO THIS ALLOWS A CERTAIN --
    -- SECTION TO HAVE SEVERAL DAYS WITHOUT CONFLICT --
    CREATE TABLE ClassEvent (
        professorID   INT          NOT NULL,
        courseID      INT          NOT NULL,
        year          INT          NOT NULL,
        term          TEXT         NOT NULL,
        sectionNumber INT          NOT NULL,
        day           TEXT, 
        startTime     TIME,
        endTime       TIME,
        location      TEXT,
        campus        TEXT,
        CHECK(day = 'Monday' OR day = 'Tuesday' OR day = 'Wednesday' OR day = 'Thursday' OR day = 'Friday' OR day = 'Saturday' OR day = 'Sunday' OR day IS NULL),
        CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
        CHECK(campus = 'Main' OR campus = 'Online' OR campus = 'Italy'),
        PRIMARY KEY(professorID, courseID, year, term, sectionNumber, day, startTime, endTime),
        FOREIGN KEY(professorID) REFERENCES Professors(professorID),
        FOREIGN KEY(courseID, year, term, sectionNumber) REFERENCES Sections(courseID, year, term, sectionNumber)
    );
    
    -- GENERATES THE PREREQUESITES --
    CREATE TABLE Prerequisites (
        courseID      INT        NOT NULL,
        year          INT        NOT NULL,
        term          TEXT       NOT NULL,
        prereqID      INT        NOT NULL,
        CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
        PRIMARY KEY(courseID, year, term, prereqID),
        FOREIGN KEY(courseID, year, term) REFERENCES ClassesAvailable(courseID, year, term),
        FOREIGN KEY(prereqID) REFERENCES Courses(courseID)
    );
    
    
    -- HOLDS THE STUDENTS THAT WILL BE TAKING THE CLASSES --
    -- SUBTYPE OF PEOPLE --
    CREATE TABLE Students (
        studentID     INT  UNIQUE NOT NULL,
        gradYear      INT         NOT NULL,
        creditsEarned INT         NOT NULL,
        PRIMARY KEY(studentID),
        FOREIGN KEY(studentID) REFERENCES People(pid)
    );
    
    -- HOLDS A CLASS RECORD FOR STUDENTS (AND POSSIBLY PROFESSORS) --
    CREATE TABLE Enrollment (
        studentID     INT         NOT NULL,
        courseID      INT         NOT NULL,
        year          INT         NOT NULL,
        term          TEXT        NOT NULL,
        sectionNumber INT         NOT NULL,
        CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
        PRIMARY KEY(studentID, courseID, year, term, sectionNumber),
        FOREIGN KEY(studentID) REFERENCES Students(studentID),
        FOREIGN KEY(courseID, year, term, sectionNumber) REFERENCES Sections(courseID, year, term, sectionNumber)
    );
    
    -- HOLDS THE DIFFERENT DEGREES THAT CAN BE ATTAINED AT THE COLLEGE/UNIVERSITY --
    CREATE TABLE Degrees (
        degreeID     SERIAL       UNIQUE NOT NULL,
        degreeName   TEXT                NOT NULL,
        degreeType   TEXT                NOT NULL,
        degDepartment VARCHAR(4)         NOT NULL,
        CHECK(degreeType = 'Major' OR degreeType = 'Minor' OR degreeType = 'Masters'),
        PRIMARY KEY(degreeID)
    );
    
    -- HOLDS THE CLASSES THAT WILL MAKE UP A DEGREE --
    CREATE TABLE DegreeReq (
        degreeID INT REFERENCES Degrees(degreeID) NOT NULL,
        courseID INT REFERENCES Courses(courseID) NOT NULL,
        PRIMARY KEY(degreeID, courseID)
    );
    
    -- HOLDS THE INSTANCE OF A DEGREE FOR A CERTAIN STUDENT --
    -- FOR EXAMPLE: A STUDENT CAN HAVE A MAJOR AND A MINOR --
    -- SO HE/SHE CAN STORE THEM SEPARATELY --
    CREATE TABLE DegreeInstance (
        degreeID        INT  REFERENCES Degrees(degreeID)   UNIQUE NOT NULL,
        studentID       INT  REFERENCES Students(studentID) UNIQUE NOT NULL,
        startDate       DATE                                       NOT NULL,
        endDate         DATE                                       NOT NULL,
        creditsRequired INT                                        NOT NULL, 
        PRIMARY KEY(degreeID, studentID)
    );
    
    -- HOLDS ALL THE RATE MY PROFESSOR STATS --
    CREATE TABLE Rating (
        professorID      INT        UNIQUE NOT NULL,
        rmpID            BIGINT     UNIQUE NOT NULL,
        avgRating        FLOAT             NOT NULL,
        avgHelpfulness   FLOAT             NOT NULL,
        avgClarity       FLOAT             NOT NULL,
        avgEasiness      FLOAT             NOT NULL,
        PRIMARY KEY(professorID, rmpID),
        FOREIGN KEY(professorID) REFERENCES Professors(professorID)
    );
    
    -- HOLDS CLASS RECORDS FOR STUDENTS --
    CREATE TABLE ClassRecord (
        studentID  INT  NOT NULL,
        courseID   INT  NOT NULL,
        year       INT  NOT NULL,
        term       TEXT NOT NULL,
        grade      TEXT NOT NULL,
        CHECK(grade = 'A' OR grade = 'A-' OR grade = 'B+' OR grade = 'B' OR grade = 'B-' OR grade = 'C+' OR grade = 'C' OR grade = 'C-' OR grade = 'D+' OR grade = 'D' OR grade = 'D-' OR grade = 'F' OR grade = 'P'),
        PRIMARY KEY(studentID, courseID, year, term, grade),
        FOREIGN KEY(courseID, year, term) REFERENCES ClassesAvailable(courseID, year, term),
        FOREIGN KEY(studentID) REFERENCES Students(studentID)
    );

有什么建议吗？

Answer 1

您可以使用INTO将结果分配给变量。

这样您就可以将两个 select 语句分配给一个变量，并在 CASE 语句中比较它们。

-- WILL SHOW WHAT STUDENTS NEED OVERRIDES FOR WHICH CLASS --
CREATE OR REPLACE FUNCTION startOverrides(REFCURSOR) RETURNS refcursor AS
$$
DECLARE
    class_size    INT := 0;
    class_members INT := 0;
    overrides     REFCURSOR  := $1;
BEGIN
    SELECT students_enrolled.courseName, COUNT(*)
    FROM students_enrolled
    GROUP BY students_enrolled.courseName, COUNT(*) INTO class_members;

    SELECT Sections.classSize
    FROM Sections
    INNER JOIN Enrollment ON Enrollment.courseID = Sections.courseID
    WHERE Enrollment.sectionNumber = Sections.sectionNumber
    INTO class_members;

    IF class_size >= class_members THEN
    OPEN overrides FOR
            SELECT students_enrolled.fname, students_enrolled.lname, students_enrolled.courseName, SUM(Sections.classSizeCap - Sections.classSize) AS overrides
             FROM students_enrolled
             INNER JOIN Sections ON Sections.courseID = students_enrolled.courseID
                                 AND Sections.sectionNumber = students_enrolled.sectionNumber
             GROUP BY students_enrolled.fname, students_enrolled.lname, students_enrolled.courseName, SUM(Sections.classSizeCap - Sections.classSize)
             ;
    END IF;
    RETURN overrids;
END
$$
LANGUAGE plpgsql;