How to implement a 'WHERE EXISTS' clause in SQL SELECT statements? - sql

Question

I need to make sure that the query listed below is implemented as a nested query with EXISTS/NOT EXISTS clauses.
"Find the first and last name of drivers who performed at least one trip in 2017."
Here is my code so far:
SELECT EMPLOYEE.FNAME, EMPLOYEE.LNAME
FROM EMPLOYEE
WHERE EXISTS
(SELECT ENUM FROM DRIVER WHERE EMPLOYEE.ENUM=DRIVER.ENUM) AND
(SELECT LNUM FROM TRIP WHERE TRIP.LNUM=DRIVER.LNUM AND
YEAR(TDATE)=2017);
I am not sure whether or not it is a syntax error or whether I have entered the wrong values. However, I triple checked all the values and they seem to match the table data.
Below I have listed the tables:
CREATE TABLE TRIP(
TNUM DECIMAL(10) NOT NULL,
LNUM DECIMAL(8) NOT NULL,
REGNUM VARCHAR(10) NOT NULL,
TDATE DATE NOT NULL,
CONSTRAINT TRIP_PKEY PRIMARY KEY (TNUM),
CONSTRAINT TRIP_CKEY UNIQUE (LNUM, REGNUM, TDATE),
CONSTRAINT TRIP_FKEY1 FOREIGN KEY (LNUM) REFERENCES DRIVER(LNUM),
CONSTRAINT TRIP_FKEY2 FOREIGN KEY (REGNUM) REFERENCES TRUCK(REGNUM) );
CREATE TABLE DRIVER(
ENUM DECIMAL(12) NOT NULL,
LNUM DECIMAL(8) NOT NULL,
STATUS VARCHAR(10) NOT NULL,
CONSTRAINT DRIVER_PKEY PRIMARY KEY(ENUM),
CONSTRAINT DRIVER_UNIQUE UNIQUE(LNUM),
CONSTRAINT DRIVER_FKEY FOREIGN KEY(ENUM) REFERENCES EMPLOYEE(ENUM),
CONSTRAINT DRIVER_STATUS CHECK ( STATUS IN ('AVAILABLE', 'BUSY', 'ON
LEAVE')) );
CREATE TABLE EMPLOYEE(
ENUM DECIMAL(12) NOT NULL,
FNAME VARCHAR(50) NOT NULL,
INITIALS VARCHAR(5) NULL,
LNAME VARCHAR(50) NOT NULL,
DOB DATE NULL,
BLDG DECIMAL(3) NOT NULL,
STREET VARCHAR(50) NOT NULL,
SUBURB VARCHAR(50) NOT NULL,
STATE VARCHAR(5) NOT NULL,
ZIPCODE DECIMAL(4) NOT NULL,
CONSTRAINT EMPLOYEE_PKEY PRIMARY KEY(ENUM) );
Below I have listed the table data:
INSERT INTO TRIP VALUES( 1, 10001, 'PKR768', '2015-01-12' );
INSERT INTO TRIP VALUES( 2, 10001, 'SYF777', '2015-02-20' );
INSERT INTO TRIP VALUES( 3, 10001, 'KKK007', '2015-03-12' );
INSERT INTO TRIP VALUES( 4, 10001, 'PKR768', '2015-06-29' );
INSERT INTO TRIP VALUES( 5, 20002, 'PKR768', '2015-01-12' );
INSERT INTO TRIP VALUES( 6, 10002, 'SYF777', '2015-02-20' );
INSERT INTO TRIP VALUES( 7, 30005, 'KKK007', '2015-03-12' );
INSERT INTO TRIP VALUES( 8, 10001, 'PKR768', '2015-01-13' );
INSERT INTO TRIP VALUES( 9, 10002, 'QRT834', '2015-09-17' );
INSERT INTO TRIP VALUES(10, 30005, 'KKK007', '2015-12-15' );
INSERT INTO TRIP VALUES(11, 10003, 'SST005', '2016-01-23' );
INSERT INTO TRIP VALUES(12, 10002, 'PKR768', '2016-03-12' );
INSERT INTO TRIP VALUES(13, 20002, 'QRT834', '2015-04-23' );
INSERT INTO TRIP VALUES(14, 20002, 'PKR008', '2015-04-23' );
INSERT INTO TRIP VALUES(15, 30005, 'PKR768', '2015-05-24' );
INSERT INTO TRIP VALUES(16, 30005, 'SST005', '2014-08-02' );
INSERT INTO TRIP VALUES(17, 20002, 'QRT834', '2014-09-17' );
INSERT INTO TRIP VALUES(18, 10001, 'KKK007', '2014-12-15' );
INSERT INTO TRIP VALUES(19, 30005, 'SST005', '2016-01-23' );
INSERT INTO TRIP VALUES(20, 10003, 'PKR768', '2016-03-12' );
INSERT INTO TRIP VALUES(21, 10001, 'QRT834', '2012-04-23' );
INSERT INTO TRIP VALUES(22, 30005, 'PKR008', '2012-04-23' );
INSERT INTO TRIP VALUES(23, 10003, 'PKR768', '2012-05-25' );
INSERT INTO TRIP VALUES(24, 20002, 'SST005', '2012-08-02' );
INSERT INTO TRIP VALUES(25, 10001, 'PKR768', '2014-01-12' );
INSERT INTO TRIP VALUES(26, 10001, 'SYF777', '2013-02-20' );
INSERT INTO TRIP VALUES(27, 20002, 'KKK007', '2013-03-12' );
INSERT INTO TRIP VALUES(28, 30005, 'PKR768', '2010-06-29' );
INSERT INTO TRIP VALUES(29, 10001, 'QRT834', '2010-09-17' );
INSERT INTO TRIP VALUES(30, 10002, 'KKK007', '2010-12-15' );
INSERT INTO TRIP VALUES(31, 10003, 'SST005', '2010-01-23' );
INSERT INTO TRIP VALUES(32, 20002, 'PKR768', '2010-03-12' );
INSERT INTO TRIP VALUES(33, 30005, 'QRT834', '2003-04-23' );
INSERT INTO TRIP VALUES(34, 30005, 'PKR008', '2004-04-23' );
INSERT INTO TRIP VALUES(35, 10001, 'PKR768', '2017-05-24' );
INSERT INTO DRIVER VALUES( 1, 10001, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 2, 10008, 'ON LEAVE' );
INSERT INTO DRIVER VALUES( 3, 10002, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 4, 10004, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 5, 10003, 'ON LEAVE' );
INSERT INTO DRIVER VALUES( 6, 10012, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 7, 20002, 'BUSY' );
INSERT INTO DRIVER VALUES( 8, 20003, 'BUSY' );
INSERT INTO DRIVER VALUES( 9, 30005, 'BUSY' );
INSERT INTO DRIVER VALUES( 10, 40002, 'BUSY' );
INSERT INTO DRIVER VALUES( 11, 20045, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 12, 20055, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 13, 20065, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 14, 10305, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 15, 10345, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 16, 10705, 'AVAILABLE' );
INSERT INTO DRIVER VALUES( 17, 40005, 'AVAILABLE' );
INSERT INTO EMPLOYEE VALUES( 1, 'John', NULL, 'Smith', NULL, 42,
'Victoria St.', 'Hurstville', 'NSW', 2456 );
INSERT INTO EMPLOYEE VALUES( 2, 'Peter', NULL, 'Taylor', '1970-01-
12', 42, 'Victoria St.', 'Hurstville', 'NSW', 2456 );
INSERT INTO EMPLOYEE VALUES( 3, 'John', NULL, 'Doe', '1966-03-23',
12, 'Station St.', 'Dapto', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 4, 'John', NULL, 'Gray', '1988-05-05',
16, 'Station St.', 'Dapto', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 5, 'Adam', NULL, 'Taylor', '1980-01-01',
42, 'Church St.', 'City', 'NSW', 2300 );
INSERT INTO EMPLOYEE VALUES( 6, 'Michael', NULL, 'Jones', '1975-03-
05', 23, 'Waterloo Ave.', 'Surry Hills', 'NSW', 2502 );
INSERT INTO EMPLOYEE VALUES( 7, 'Frederic', NULL, 'Jones', NULL, 3,
'Victoria St.', 'Redfern', 'NSW', 2420 );
INSERT INTO EMPLOYEE VALUES( 8, 'Peter', NULL, 'O''Brien', '1983-02-
28', 19, 'Lucas Dr.', 'Horsley', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 9, 'John', NULL, 'Lucas', '1966-12-16',
20, 'Huxley St.', 'Horsley', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 10, 'John', NULL, 'Fox', '1975-10-15',
18, 'Victoria St.', 'Hurstville', 'NSW', 2456 );
INSERT INTO EMPLOYEE VALUES( 11, 'Adam', NULL, 'Fox', NULL, 45,
'Victoria St.', 'Hurstville', 'NSW', 2456 );
INSERT INTO EMPLOYEE VALUES( 12, 'Phillip', NULL, ',Cox', '1984-12-
12', 5, 'The Avenue', 'Rockdale', 'NSW', 2300 );
INSERT INTO EMPLOYEE VALUES( 13, 'Andrew', 'K', 'Smith', '1969-04-
04', 42, 'Bambaramba Ave.', 'Pennant Hills', 'NSW', 2556 );
INSERT INTO EMPLOYEE VALUES( 14, 'Andrew', 'R', 'Smith', '1992-04-
01', 67, 'King Cr.', 'Hurstville', 'NSW', 2456 );
INSERT INTO EMPLOYEE VALUES( 15, 'Michael', NULL, 'Potter', '1995-04-
01', 568, 'Bong Bong St.', 'Horsley', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 16, 'Harry', NULL, 'Potter', '1995-04-
01', 568, 'Bong Bong St.', 'Horsley', 'NSW', 2530 );
INSERT INTO EMPLOYEE VALUES( 17, 'James', NULL, 'Bond', NULL, 007,
'Alan Bond St.', 'Perth', 'WA', 6000 );
INSERT INTO EMPLOYEE VALUES( 18, 'Paris', NULL, 'Hilton', NULL, 1,
'Hilton St.', 'Melbourne', 'Vic', 3000 );
INSERT INTO EMPLOYEE VALUES( 19, 'Lady', NULL, 'Gaga', NULL, 3, 'Pork
st.', 'Hobart', 'Tas', 7000 );
INSERT INTO EMPLOYEE VALUES( 20, 'Robin', NULL, 'Hood', NULL, 6,
'Nottingham Pl.', 'Sydney', 'NSW', 2000 );

Answer 1

The correct syntax is:
SELECT E.FNAME, E.LNAME
FROM EMPLOYEE E
WHERE EXISTS (SELECT 1
FROM DRIVER D
WHERE D.ENUM = E.ENUM
) AND
EXISTS (SELECT 1
FROM TRIP T
WHERE T.LNUM = E.LNUM AND YEAR(T.TDATE) = 2017
);
You need to repeat the EXISTS. In addition:
Use table aliases and qualify all column references. This is a best practice, so get used to it now when you are learning SQL.
The value in the SELECT for an EXISTS is irrelevant. I use 1 because it is easy to type.
Not all databases support YEAR(). The standard syntax is EXTRACT(YEAR FROM T.TDATE). I assume your database does support the function you are using.
I doubt that you really need to compare to DRIVERS (in the real world). Wouldn't a drive on a trip automatically be a "driver"?