Write a Postgre SQL subquery to find the first_name and last_name of the employees who are not supervisors

SELECT b.first_name,b.last_name 
FROM employees b 
WHERE NOT EXISTS (
SELECT 'X' 
FROM employees a 
WHERE a.manager_id = b.employee_id);

Sample table: employees

+-------------+-------------+-------------+----------+--------------------+------------+------------+----------+----------------+------------+---------------+
| EMPLOYEE_ID | FIRST_NAME  | LAST_NAME   | EMAIL    | PHONE_NUMBER       | HIRE_DATE  | JOB_ID     | SALARY   | COMMISSION_PCT | MANAGER_ID | DEPARTMENT_ID |
+-------------+-------------+-------------+----------+--------------------+------------+------------+----------+----------------+------------+---------------+
|         100 | Steven      | King        | SKING    | 515.123.4567       | 1987-06-17 | AD_PRES    | 24000.00 |           0.00 |          0 |   		  90 |
|         101 | Neena       | Kochhar     | NKOCHHAR | 515.123.4568       | 1987-06-18 | AD_VP      | 17000.00 |           0.00 |        100 |            90 |
|         102 | Lex         | De Haan     | LDEHAAN  | 515.123.4569       | 1987-06-19 | AD_VP      | 17000.00 |           0.00 |        100 |            90 |
|         103 | Alexander   | Hunold      | AHUNOLD  | 590.423.4567       | 1987-06-20 | IT_PROG    |  9000.00 |           0.00 |        102 |            60 |
|         104 | Bruce       | Ernst       | BERNST   | 590.423.4568       | 1987-06-21 | IT_PROG    |  6000.00 |           0.00 |        103 |            60 |
|         105 | David       | Austin      | DAUSTIN  | 590.423.4569       | 1987-06-22 | IT_PROG    |  4800.00 |           0.00 |        103 |            60 |
|         106 | Valli       | Pataballa   | VPATABAL | 590.423.4560       | 1987-06-23 | IT_PROG    |  4800.00 |           0.00 |        103 |            60 |
|         107 | Diana       | Lorentz     | DLORENTZ | 590.423.5567       | 1987-06-24 | IT_PROG    |  4200.00 |           0.00 |        103 |            60 |
|         108 | Nancy       | Greenberg   | NGREENBE | 515.124.4569       | 1987-06-25 | FI_MGR     | 12000.00 |           0.00 |        101 |           100 |
|         109 | Daniel      | Faviet      | DFAVIET  | 515.124.4169       | 1987-06-26 | FI_ACCOUNT |  9000.00 |           0.00 |        108 |           100 |
|         110 | John        | Chen        | JCHEN    | 515.124.4269       | 1987-06-27 | FI_ACCOUNT |  8200.00 |           0.00 |        108 |           100 |
|         111 | Ismael      | Sciarra     | ISCIARRA | 515.124.4369       | 1987-06-28 | FI_ACCOUNT |  7700.00 |           0.00 |        108 |           100 |
|         112 | Jose Manuel | Urman       | JMURMAN  | 515.124.4469       | 1987-06-29 | FI_ACCOUNT |  7800.00 |           0.00 |        108 |           100 |
|         113 | Luis        | Popp        | LPOPP    | 515.124.4567       | 1987-06-30 | FI_ACCOUNT |  6900.00 |           0.00 |        108 |           100 |
|         114 | Den         | Raphaely    | DRAPHEAL | 515.127.4561       | 1987-07-01 | PU_MAN     | 11000.00 |           0.00 |        100 |            30 |
|         115 | Alexander   | Khoo        | AKHOO    | 515.127.4562       | 1987-07-02 | PU_CLERK   |  3100.00 |           0.00 |        114 |            30 |
|         116 | Shelli      | Baida       | SBAIDA   | 515.127.4563       | 1987-07-03 | PU_CLERK   |  2900.00 |           0.00 |        114 |            30 |
|         117 | Sigal       | Tobias      | STOBIAS  | 515.127.4564       | 1987-07-04 | PU_CLERK   |  2800.00 |           0.00 |        114 |            30 |
|         118 | Guy         | Himuro      | GHIMURO  | 515.127.4565       | 1987-07-05 | PU_CLERK   |  2600.00 |           0.00 |        114 |            30 |
|         119 | Karen       | Colmenares  | KCOLMENA | 515.127.4566       | 1987-07-06 | PU_CLERK   |  2500.00 |           0.00 |        114 |            30 |
|         120 | Matthew     | Weiss       | MWEISS   | 650.123.1234       | 1987-07-07 | ST_MAN     |  8000.00 |           0.00 |        100 |            50 |
|         121 | Adam        | Fripp       | AFRIPP   | 650.123.2234       | 1987-07-08 | ST_MAN     |  8200.00 |           0.00 |        100 |            50 |
|         122 | Payam       | Kaufling    | PKAUFLIN | 650.123.3234       | 1987-07-09 | ST_MAN     |  7900.00 |           0.00 |        100 |            50 |
|         123 | Shanta      | Vollman     | SVOLLMAN | 650.123.4234       | 1987-07-10 | ST_MAN     |  6500.00 |           0.00 |        100 |            50 |
|         124 | Kevin       | Mourgos     | KMOURGOS | 650.123.5234       | 1987-07-11 | ST_MAN     |  5800.00 |           0.00 |        100 |            50 |
|         125 | Julia       | Nayer       | JNAYER   | 650.124.1214       | 1987-07-12 | ST_CLERK   |  3200.00 |           0.00 |        120 |            50 |
|         126 | Irene       | Mikkilineni | IMIKKILI | 650.124.1224       | 1987-07-13 | ST_CLERK   |  2700.00 |           0.00 |        120 |            50 |
|         127 | James       | Landry      | JLANDRY  | 650.124.1334       | 1987-07-14 | ST_CLERK   |  2400.00 |           0.00 |        120 |            50 |
|         128 | Steven      | Markle      | SMARKLE  | 650.124.1434       | 1987-07-15 | ST_CLERK   |  2200.00 |           0.00 |        120 |            50 |
|         129 | Laura       | Bissot      | LBISSOT  | 650.124.5234       | 1987-07-16 | ST_CLERK   |  3300.00 |           0.00 |        121 |            50 |
|         130 | Mozhe       | Atkinson    | MATKINSO | 650.124.6234       | 1987-07-17 | ST_CLERK   |  2800.00 |           0.00 |        121 |            50 |
|         131 | James       | Marlow      | JAMRLOW  | 650.124.7234       | 1987-07-18 | ST_CLERK   |  2500.00 |           0.00 |        121 |            50 |
|         132 | TJ          | Olson       | TJOLSON  | 650.124.8234       | 1987-07-19 | ST_CLERK   |  2100.00 |           0.00 |        121 |            50 |
|         133 | Jason       | Mallin      | JMALLIN  | 650.127.1934       | 1987-07-20 | ST_CLERK   |  3300.00 |           0.00 |        122 |            50 |
|         134 | Michael     | Rogers      | MROGERS  | 650.127.1834       | 1987-07-21 | ST_CLERK   |  2900.00 |           0.00 |        122 |            50 |
|         135 | Ki          | Gee         | KGEE     | 650.127.1734       | 1987-07-22 | ST_CLERK   |  2400.00 |           0.00 |        122 |            50 |
|         136 | Hazel       | Philtanker  | HPHILTAN | 650.127.1634       | 1987-07-23 | ST_CLERK   |  2200.00 |           0.00 |        122 |            50 |
|         137 | Renske      | Ladwig      | RLADWIG  | 650.121.1234       | 1987-07-24 | ST_CLERK   |  3600.00 |           0.00 |        123 |            50 |
|         138 | Stephen     | Stiles      | SSTILES  | 650.121.2034       | 1987-07-25 | ST_CLERK   |  3200.00 |           0.00 |        123 |            50 |
|         139 | John        | Seo         | JSEO     | 650.121.2019       | 1987-07-26 | ST_CLERK   |  2700.00 |           0.00 |        123 |            50 |
|         140 | Joshua      | Patel       | JPATEL   | 650.121.1834       | 1987-07-27 | ST_CLERK   |  2500.00 |           0.00 |        123 |            50 |
|         141 | Trenna      | Rajs        | TRAJS    | 650.121.8009       | 1987-07-28 | ST_CLERK   |  3500.00 |           0.00 |        124 |            50 |
|         142 | Curtis      | Davies      | CDAVIES  | 650.121.2994       | 1987-07-29 | ST_CLERK   |  3100.00 |           0.00 |        124 |            50 |
|         143 | Randall     | Matos       | RMATOS   | 650.121.2874       | 1987-07-30 | ST_CLERK   |  2600.00 |           0.00 |        124 |            50 |
|         144 | Peter       | Vargas      | PVARGAS  | 650.121.2004       | 1987-07-31 | ST_CLERK   |  2500.00 |           0.00 |        124 |            50 |
|         145 | John        | Russell     | JRUSSEL  | 011.44.1344.429268 | 1987-08-01 | SA_MAN     | 14000.00 |           0.40 |        100 |            80 |
|         146 | Karen       | Partners    | KPARTNER | 011.44.1344.467268 | 1987-08-02 | SA_MAN     | 13500.00 |           0.30 |        100 |            80 |
|         147 | Alberto     | Errazuriz   | AERRAZUR | 011.44.1344.429278 | 1987-08-03 | SA_MAN     | 12000.00 |           0.30 |        100 |            80 |
|         148 | Gerald      | Cambrault   | GCAMBRAU | 011.44.1344.619268 | 1987-08-04 | SA_MAN     | 11000.00 |           0.30 |        100 |            80 |
|         149 | Eleni       | Zlotkey     | EZLOTKEY | 011.44.1344.429018 | 1987-08-05 | SA_MAN     | 10500.00 |           0.20 |        100 |            80 |
|         150 | Peter       | Tucker      | PTUCKER  | 011.44.1344.129268 | 1987-08-06 | SA_REP     | 10000.00 |           0.30 |        145 |            80 |
|         151 | David       | Bernstein   | DBERNSTE | 011.44.1344.345268 | 1987-08-07 | SA_REP     |  9500.00 |           0.25 |        145 |            80 |
|         152 | Peter       | Hall        | PHALL    | 011.44.1344.478968 | 1987-08-08 | SA_REP     |  9000.00 |           0.25 |        145 |            80 |
|         153 | Christopher | Olsen       | COLSEN   | 011.44.1344.498718 | 1987-08-09 | SA_REP     |  8000.00 |           0.20 |        145 |            80 |
|         154 | Nanette     | Cambrault   | NCAMBRAU | 011.44.1344.987668 | 1987-08-10 | SA_REP     |  7500.00 |           0.20 |        145 |            80 |
|         155 | Oliver      | Tuvault     | OTUVAULT | 011.44.1344.486508 | 1987-08-11 | SA_REP     |  7000.00 |           0.15 |        145 |            80 |
|         156 | Janette     | King        | JKING    | 011.44.1345.429268 | 1987-08-12 | SA_REP     | 10000.00 |           0.35 |        146 |            80 |
|         157 | Patrick     | Sully       | PSULLY   | 011.44.1345.929268 | 1987-08-13 | SA_REP     |  9500.00 |           0.35 |        146 |            80 |
|         158 | Allan       | McEwen      | AMCEWEN  | 011.44.1345.829268 | 1987-08-14 | SA_REP     |  9000.00 |           0.35 |        146 |            80 |
|         159 | Lindsey     | Smith       | LSMITH   | 011.44.1345.729268 | 1987-08-15 | SA_REP     |  8000.00 |           0.30 |        146 |            80 |
|         160 | Louise      | Doran       | LDORAN   | 011.44.1345.629268 | 1987-08-16 | SA_REP     |  7500.00 |           0.30 |        146 |            80 |
|         161 | Sarath      | Sewall      | SSEWALL  | 011.44.1345.529268 | 1987-08-17 | SA_REP     |  7000.00 |           0.25 |        146 |            80 |
|         162 | Clara       | Vishney     | CVISHNEY | 011.44.1346.129268 | 1987-08-18 | SA_REP     | 10500.00 |           0.25 |        147 |            80 |
|         163 | Danielle    | Greene      | DGREENE  | 011.44.1346.229268 | 1987-08-19 | SA_REP     |  9500.00 |           0.15 |        147 |            80 |
|         164 | Mattea      | Marvins     | MMARVINS | 011.44.1346.329268 | 1987-08-20 | SA_REP     |  7200.00 |           0.10 |        147 |            80 |
|         165 | David       | Lee         | DLEE     | 011.44.1346.529268 | 1987-08-21 | SA_REP     |  6800.00 |           0.10 |        147 |            80 |
|         166 | Sundar      | Ande        | SANDE    | 011.44.1346.629268 | 1987-08-22 | SA_REP     |  6400.00 |           0.10 |        147 |            80 |
|         167 | Amit        | Banda       | ABANDA   | 011.44.1346.729268 | 1987-08-23 | SA_REP     |  6200.00 |           0.10 |        147 |            80 |
|         168 | Lisa        | Ozer        | LOZER    | 011.44.1343.929268 | 1987-08-24 | SA_REP     | 11500.00 |           0.25 |        148 |            80 |
|         169 | Harrison    | Bloom       | HBLOOM   | 011.44.1343.829268 | 1987-08-25 | SA_REP     | 10000.00 |           0.20 |        148 |            80 |
|         170 | Tayler      | Fox         | TFOX     | 011.44.1343.729268 | 1987-08-26 | SA_REP     |  9600.00 |           0.20 |        148 |            80 |
|         171 | William     | Smith       | WSMITH   | 011.44.1343.629268 | 1987-08-27 | SA_REP     |  7400.00 |           0.15 |        148 |            80 |
|         172 | Elizabeth   | Bates       | EBATES   | 011.44.1343.529268 | 1987-08-28 | SA_REP     |  7300.00 |           0.15 |        148 |            80 |
|         173 | Sundita     | Kumar       | SKUMAR   | 011.44.1343.329268 | 1987-08-29 | SA_REP     |  6100.00 |           0.10 |        148 |            80 |
|         174 | Ellen       | Abel        | EABEL    | 011.44.1644.429267 | 1987-08-30 | SA_REP     | 11000.00 |           0.30 |        149 |            80 |
|         175 | Alyssa      | Hutton      | AHUTTON  | 011.44.1644.429266 | 1987-08-31 | SA_REP     |  8800.00 |           0.25 |        149 |            80 |
|         176 | Jonathon    | Taylor      | JTAYLOR  | 011.44.1644.429265 | 1987-09-01 | SA_REP     |  8600.00 |           0.20 |        149 |            80 |
|         177 | Jack        | Livingston  | JLIVINGS | 011.44.1644.429264 | 1987-09-02 | SA_REP     |  8400.00 |           0.20 |        149 |            80 |
|         178 | Kimberely   | Grant       | KGRANT   | 011.44.1644.429263 | 1987-09-03 | SA_REP     |  7000.00 |           0.15 |        149 |             0 |
|         179 | Charles     | Johnson     | CJOHNSON | 011.44.1644.429262 | 1987-09-04 | SA_REP     |  6200.00 |           0.10 |        149 |            80 |
|         180 | Winston     | Taylor      | WTAYLOR  | 650.507.9876       | 1987-09-05 | SH_CLERK   |  3200.00 |           0.00 |        120 |            50 |
|         181 | Jean        | Fleaur      | JFLEAUR  | 650.507.9877       | 1987-09-06 | SH_CLERK   |  3100.00 |           0.00 |        120 |            50 |
|         182 | Martha      | Sullivan    | MSULLIVA | 650.507.9878       | 1987-09-07 | SH_CLERK   |  2500.00 |           0.00 |        120 |            50 |
|         183 | Girard      | Geoni       | GGEONI   | 650.507.9879       | 1987-09-08 | SH_CLERK   |  2800.00 |           0.00 |        120 |            50 |
|         184 | Nandita     | Sarchand    | NSARCHAN | 650.509.1876       | 1987-09-09 | SH_CLERK   |  4200.00 |           0.00 |        121 |            50 |
|         185 | Alexis      | Bull        | ABULL    | 650.509.2876       | 1987-09-10 | SH_CLERK   |  4100.00 |           0.00 |        121 |            50 |
|         186 | Julia       | Dellinger   | JDELLING | 650.509.3876       | 1987-09-11 | SH_CLERK   |  3400.00 |           0.00 |        121 |            50 |
|         187 | Anthony     | Cabrio      | ACABRIO  | 650.509.4876       | 1987-09-12 | SH_CLERK   |  3000.00 |           0.00 |        121 |            50 |
|         188 | Kelly       | Chung       | KCHUNG   | 650.505.1876       | 1987-09-13 | SH_CLERK   |  3800.00 |           0.00 |        122 |            50 |
|         189 | Jennifer    | Dilly       | JDILLY   | 650.505.2876       | 1987-09-14 | SH_CLERK   |  3600.00 |           0.00 |        122 |            50 |
|         190 | Timothy     | Gates       | TGATES   | 650.505.3876       | 1987-09-15 | SH_CLERK   |  2900.00 |           0.00 |        122 |            50 |
|         191 | Randall     | Perkins     | RPERKINS | 650.505.4876       | 1987-09-16 | SH_CLERK   |  2500.00 |           0.00 |        122 |            50 |
|         192 | Sarah       | Bell        | SBELL    | 650.501.1876       | 1987-09-17 | SH_CLERK   |  4000.00 |           0.00 |        123 |            50 |
|         193 | Britney     | Everett     | BEVERETT | 650.501.2876       | 1987-09-18 | SH_CLERK   |  3900.00 |           0.00 |        123 |            50 |
|         194 | Samuel      | McCain      | SMCCAIN  | 650.501.3876       | 1987-09-19 | SH_CLERK   |  3200.00 |           0.00 |        123 |            50 |
|         195 | Vance       | Jones       | VJONES   | 650.501.4876       | 1987-09-20 | SH_CLERK   |  2800.00 |           0.00 |        123 |            50 |
|         196 | Alana       | Walsh       | AWALSH   | 650.507.9811       | 1987-09-21 | SH_CLERK   |  3100.00 |           0.00 |        124 |            50 |
|         197 | Kevin       | Feeney      | KFEENEY  | 650.507.9822       | 1987-09-22 | SH_CLERK   |  3000.00 |           0.00 |        124 |            50 |
|         198 | Donald      | OConnell    | DOCONNEL | 650.507.9833       | 1987-09-23 | SH_CLERK   |  2600.00 |           0.00 |        124 |            50 |
|         199 | Douglas     | Grant       | DGRANT   | 650.507.9844       | 1987-09-24 | SH_CLERK   |  2600.00 |           0.00 |        124 |            50 |
|         200 | Jennifer    | Whalen      | JWHALEN  | 515.123.4444       | 1987-09-25 | AD_ASST    |  4400.00 |           0.00 |        101 |            10 |
|         201 | Michael     | Hartstein   | MHARTSTE | 515.123.5555       | 1987-09-26 | MK_MAN     | 13000.00 |           0.00 |        100 |            20 |
|         202 | Pat         | Fay         | PFAY     | 603.123.6666       | 1987-09-27 | MK_REP     |  6000.00 |           0.00 |        201 |            20 |
|         203 | Susan       | Mavris      | SMAVRIS  | 515.123.7777       | 1987-09-28 | HR_REP     |  6500.00 |           0.00 |        101 |            40 |
|         204 | Hermann     | Baer        | HBAER    | 515.123.8888       | 1987-09-29 | PR_REP     | 10000.00 |           0.00 |        101 |            70 |
|         205 | Shelley     | Higgins     | SHIGGINS | 515.123.8080       | 1987-09-30 | AC_MGR     | 12000.00 |           0.00 |        101 |           110 |
|         206 | William     | Gietz       | WGIETZ   | 515.123.8181       | 1987-10-01 | AC_ACCOUNT |  8300.00 |           0.00 |        205 |           110 |
+-------------+-------------+-------------+----------+--------------------+------------+------------+----------+----------------+------------+---------------+

Output:

pg_exercises=# SELECT b.first_name,b.last_name
pg_exercises-# FROM employees b
pg_exercises-# WHERE NOT EXISTS (
pg_exercises(# SELECT 'X'
pg_exercises(# FROM employees a
pg_exercises(# WHERE a.manager_id = b.employee_id);
 first_name  |  last_name
-------------+-------------
 Bruce       | Ernst
 David       | Austin
 Valli       | Pataballa
 Diana       | Lorentz
 Alexander   | Khoo
 Shelli      | Baida
 Sigal       | Tobias
 Guy         | Himuro
 Karen       | Colmenares
 Jason       | Mallin
 Michael     | Rogers
 Ki          | Gee
 Hazel       | Philtanker
 Stephen     | Stiles
 John        | Seo
 Joshua      | Patel
 Trenna      | Rajs
 Curtis      | Davies
 Randall     | Matos
 Peter       | Vargas
 Janette     | King
...          | ...
 Susan       | Mavris
 Hermann     | Baer
 William     | Gietz
(88 rows)