feat: add sql solution to lc problem: No.1978 (#1774)

Author: thinkasany

solution/1900-1999/1978.Employees Whose Manager Left the Company/README.md

@@ -80,4 +80,12 @@ WHERE b.employee_id IS NULL AND a.salary < 30000 AND a.manager_id IS NOT NULL
 ORDER BY a.employee_id;
 ```
 
+```sql
+# Write your MySQL query statement below
+SELECT employee_id
+FROM Employees
+WHERE salary < 30000 AND manager_id NOT IN (SELECT employee_id FROM Employees)
+ORDER BY employee_id;
+```
+
 <!-- tabs:end -->

solution/1900-1999/1978.Employees Whose Manager Left the Company/README_EN.md

@@ -72,4 +72,12 @@ WHERE b.employee_id IS NULL AND a.salary < 30000 AND a.manager_id IS NOT NULL
 ORDER BY a.employee_id;
 ```
 
+```sql
+# Write your MySQL query statement below
+SELECT employee_id
+FROM Employees
+WHERE salary < 30000 AND manager_id NOT IN (SELECT employee_id FROM Employees)
+ORDER BY employee_id;
+```
+
 <!-- tabs:end -->