查询内部联接两个表

Question

Below I have created three tables:

CREATE TABLE EMPLOYEE(
    EMPLOYEE_ID [int] identity (1,1) not null,
    FIRSTNAME [nvarchar] (20) not null,
    LASTNAME [nvarchar] (20) not null, 
);

CREATE TABLE LEAVE(
    LEAVE_ID [int] identity (1,1) not null,
    START_DATE [DATE]  not null,
    END_DATE [DATE]  not null,
);

CREATE TABLE EMPLOYEE_LEAVE(
    ELEAVE_ID [int] identity (1,1) not null,
    LEAVE_ID [int] not null,
    EMPLOYEE_ID [int] not null,
);

I'm trying to create a query that shows all the employees leave dates.

CREATE VIEW [VW_CURRENT_LEAVE]
AS
SELECT E.EMPLOYEE_ID, 
    E.FIRSTNAME, 
    E.LASTNAME, 
    L.START_DATE, 
    L.END_DATE
FROM EMPLOYEE E, 
    LEAVE L, 
    EMPLOYEE_LEAVE EL
WHERE   E.EMPLOYEE_ID = EL.E.EMPLOYEE_ID
AND EL.LEAVE_ID = L.LEAVE_ID
AND L.END_DATE >= GETDATE()
AND L.START_DATE <= GETDATE()

GO

But the query just returns everything as null. The titles are as I want them but how come it's not filling the information?

Answer 1

First of all, you should get rid of those commas in the FROM clause. This style of table join has been officially deprecated since the ANSI-92 standard, for reasons including that it makes it difficult to even figure out what the actual join criteria is.

Try this query:

SELECT E.EMPLOYEE_ID,
       E.FIRSTNAME,
       E.LASTNAME,
       L.START_DATE,
       L.END_DATE
FROM EMPLOYEE E
INNER JOIN EMPLOYEE_LEAVE EL
    ON E.EMPLOYEE_ID = EL.EMPLOYEE_ID
INNER JOIN LEAVE L
    ON EL.LEAVE_ID = L.LEAVE_ID

See what this returns, and if it looks good you can try adding the following WHERE clause:

WHERE L.END_DATE >= GETDATE() AND
      L.START_DATE <= GETDATE()

If adding the WHERE clause causes no records to be returned you should check to makes sure that any employees have active leave scheduled.