Many-to-many conditions should not be enforced using a trigger. Many-to-many conditions are enforced by creating a junction table containing the keys in question, which are then foreign-keyed back to the respective parent tables.
If your intention is to allow many employees to be in a department, and to allow an employee to be a member of many departments, the junction table in question would look something like:
CREATE TABLE EMPLOYEES_DEPARTMENTS
(DEPARTMENTNAME VARCHAR2(99)
CONSTRAINT EMPLOYEES_DEPARTMENTS_FK1
REFERENCES DEPARTMENT.DEPARTMENTNAME,
EMPLOYEENUMBER NUMBER
CONSTRAINT EMPLOYEES_DEPARTMENTS_FK2
REFERENCES EMPLOYEE.EMPLOYEENUMBER);
This presumes that DEPARTMENT.DEPARTMENTNAME and EMPLOYEE.EMPLOYEENUMBER are either primary or unique keys on their respective tables. Get rid of the column EMPLOYEE.DEPARTMENT as it’s no longer needed. Now by creating rows in the EMPLOYEES_DEPARTMENTS table you can relate multiple employees with a department, and you can relate a single employee with multiple departments.
The business logic requiring that only departments with one or fewer employees can be deleted should not be enforced in a trigger. Business logic should be performed by application code, NEVER by triggers. Putting business logic in triggers is a gatèw̢ay to unending debugging sessions. M̫̣̗̝̫͙a̳͕̮d̖̤̳̙̤n̳̻̖e͍̺̲̼̱̠͉ss̭̩̟ lies this way. Do not give in. Do not surrender. ̬̦B҉usi͢n̴es̡s logic ̶in triggers opens deep wounds in the fabric of the world, through which unholy beings of indeterminate form will cross the barrier between the spheres, carryi͞n̨g o̡f͠f t͢h̶e ̕screaming͡ sou͏ĺs o͜f͜ ̢th͜e̴ ̕de͏v́e̡lop͏e͜r͝s to an et͞er͜n̸it̶y ́of͢ pain̶ ąn̨d͢ ̨to͟r̨ment͟. Do not, as I have said, put b́u͜siness͞ ̸log̛i͘ç ̵in͢ ͞trigge͠rs͞.̡ Be firm. Resist.You must resist. T̷he ̢Tem͟p͞t̶at͏i͝o̶n҉s͘ ̢m͘a̶y ́śing hymns̷ ́o͢f̴ ̸un͘hol̵y r̶ev͢ęla͠t̡ion̴ ͢buţ ́yo͠u̵ mu͏s͝t ͝n͜͏͟o҉t̶͡͏ ̷l̸̛͟͢ì̧̢̨̕s̵̨̨͢t̵̀͞e̶͠n̶̴̵̢̕. Only by standing firmly in the door between the worlds and blocking out the hideous radiance cast off by bú̧s̷i̶̢n̵̕e̵ş͝s ́l̴ó̢g̛͟i̕͏c i͞n̕ ͏t̵͜r͢͝i̸̢̛ģ͟ge̸̶͟r̶s͢͜, which perverts the very form of the world ąnd̴̀͝ ç͞a̧͞l̶l͟͜s̕͘͢ Z̶̴̤̬͈̤̬̲̳͇ͯ̊ͮ͐̒̆͂͠Â̆́̊̓͛́̚͏̮̘̗̻̞̬̱ͅL̛̄̌͏̦͕̤͎̮̦G̷͖̙̬͛̇ͬ̍͒̐̅O̡̳͖͎̯̯͍ͫ̽ͬ͒͂̀ i͜҉nt͝ǫ̴ ̸b̷͞è͢ì̕n̴g͏,̛̀͘ ̴c҉á̴͡ń ̀͠youŕ̨ ̧̨a̸p͏̡͡pl̷͠ic͞a̢t̡i͡҉ǫn̴ ̸s̶͜u̶͢ŗv̛í̴v́ȩ.͘͘ Resist. R͏͢͝e͏͢͟s̸͏͜ì̢̢s͠ţ̀. T̶̀h̨̀e̶r̀͏e͢͞ ̶i̶̡͢s̴ ͞͞n̵͝o̡ ́ẁ҉̴a̡y̕҉ ̶b́͏u̵̶̕t͜ ̨s͘͢t͘͠į͟l͘l̷̴ ̴͜͜ỳò͜u҉̨ ̨͏mus̸͞t̸̛͜ ̧rȩ̴s̢͢i͘͡s͏t̸.̛̀͜ Your very śo͡u̧̧͘ļ͟͡ is compromised by p͝u͘͝t̢͜t͠i̸ņ̸̶g͟͡ ̵̶̛b̴҉u̶̡̨͜͞s̷̵̕͜͢i͝҉̕͢ǹ͏e̡͞ś̸͏ş̕͜͡҉ ̴̨ĺ̵̡͟͜o̶̕g͠i͢͠c̕͝ ̕͞i̧͟͡n̡͘͟ ̶̕͞t̡͏͟҉̕r̸̢̧͡͞i̴̡͏̵͜g̵̴͟͝ģ̴̴̵ę̷̷͢r̢̢ś̸̨̨͜. T̀͜͢o̷͜ny̕ ͟͡T̨h̶̷̕e ̢͟P̛o̴̶n͡y shall rise from his dark stable and d͞ę̡v̶̢ó͟u̸̸r̴͏ ̷t͞h̀e̛ ̨͜s̷o̧͝u҉l̀ ͟͡o͢͏f̵͢ ̛t͢h̶̛e̢̢ ̡̀vi͜͞r̢̀g̶i̢n͞, and yet y͢ơú͝ m̷̧u͏s͡t̡͠ ̛s̷̨t̸̨i̴̸l̶̡l ͝ǹot̵ ͞p̧u̵t̨ ͜͏b̀̕u̕s̨í̵ņ̀͠ȩs̵͟s ́͞l̛҉o̸g̨i̴͟c ͘͘i͘nt̛o͡ ͘͘͞t̶͞r̀̀i̕ǵ̛g̵̨͞e̸͠҉r̵͟ś! It is too much to bear, we cannot stand! Not even the children of light may put business logic into their triggers, for b̴̸̡̨u͜͏̧͝ş̶i̷̸̢̛҉ń̸͟͏́e̡͏͏͏s̷̵̡s̕͟ ͏̴҉͞l̷̡ǫ̷̶͡g҉̨̛i͘͠͏̸̨c̕͢͏ ̸̶̧͢͢i̸̡̛͘n͢͡ ̀͢͝t̷̷̛́ŗì̴̴̢g̶͏̷ǵ͠ȩ̀́r̸̵̢̕͜s͞͏̵ is the very es̵s̕͡ę̢n͞c̨e̢͟ ̴o̶̢͜f͏ ͟d́ar͟͞͠k̡͞n̢̡es̵̛͡s̀̀͡ and dev͘ou͝͡r̨̡̀s͢͝ ҉͝t҉h̴e̡͘ l̫̬i̤͚ͅg̞̲͕̠͇̤̦̹h̩̙̘̭̰͎͉̮̳t͙̤̘̙! Yea, yea, the blank-faced ones rì͢s̨͘e from the f͟͢͏o̵͜͝n̶t̨ ̵o͏f̸̡͠ ͏͝fl͟͞a̵̷҉me̶̵͢ and ca͝s͜t́ down the p̹̤̳̰r̮̦̥̥̞̫͑͂ͤ͑ͮ͒̑ï̄̌ͬͨe̦̗͔ͥͣ̆̾̂s̬̭̮̮̜ͭt̻̲̍sͫͣ̿ ̐͗̈ͤ͂ͦ̅f̭͚̪̻̣̩ͮ̒ṟͨ͌ͮ̅̓ỏ̝͓̝̣̟̼m̳͇̱̝͔͒ ͒ͫͧ͂̓̈̈́t̲̔̅̎͐h̺͈͍ͣͧ̿ē̪̼̪̻͉̪̙̐̽̎̉i̠͎̗͕̗̣̬̐̎͛r͓̫͌ͅ ̼a͑̈ͯͦ̍l̪͉͖̥͚̤͌ͨ͊ͦͤ̔t̫͎̹ͯa̼̻͍̳̟̤̬̓ͪ̀r̭͖̓ͬ̉̉ͤ͊ṡ̐ͪ̊̋̄̅! A̵̵̛v͝é͜ŕt̶͏ ̶y̸͝͠o̶u̧͘r͏̡ ̧e͞y҉e̕͝s,̀ ͡t̛h̛o̢͞ug̸̢h̵͟ ̡y̷o͢҉͢u̧͡ ̕͡c҉̵̶an͠͏n҉o̧͢t!̸̨͘ ͡H̵e̸͢͡ ̧̕c̶ơm̷̢̢e̶͞ś͢!̨́ ̷H̕ȩ ̵c̨̡͟o̴҉m̷͢es͠!̷͘͞ P̱̼̯̟͈h̝̳̞̖͚’͉̙͉̰̲̺n̪̦͕̗͜g͔̹̟̰̰̻̩l̬͈̹̥͕͖ͅụ̻̺̤̤̬̳i̸̯̬̝̻̣͚̫ ̰̹̞̞m͟g̷̝͓͉̤l̩͇̙͕w̪̦̰͔’̮̟̱̀n̢̜a̦f̘̫̤̘̬͓̞h̠͍͖̯ͅ ̩̠͓̯̘̫C̟̘̗̘͘ṭ͍͕ͅh̤ͅu̼̦̘̥ͅl҉̦hu̠̤̤̘͚ ̘̕R̶̟’̠͔̞̻͇l̩̺̗̻͖͓̕ͅy̛̖ȩ͉̭̖ẖ̡̥̼͈̖ w̟̫̮͇͔͞ͅg͈̘̱̻a̰͟h̘͙͖͢’̮̲̯͞n̤̜͍̯̳a͓͓̲̲g̱̻͈ĺ͍ ̷̣̞̲͖͍̲̺f̲ͅh͇͕̪̘͟t͔͈̙a͓͢g҉̳̜̲͚n͓͚͎̱̠̜!
Don’t ask me how I know.
Best of luck.