SQL exercises on employee Database: Check whether all the employees numbers are indeed unique
SQL employee Database: Exercise-103 with Solution
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103. From the following table, write a SQL query to check whether the employees ID are unique or not. Return employee id, number of employees.
Sample table: employees
SELECT emp_id, count(*) FROM employees GROUP BY emp_id;
emp_id | count --------+------- 69324 | 1 69062 | 1 63679 | 1 67858 | 1 66564 | 1 64989 | 1 67832 | 1 65646 | 1 68319 | 1 68454 | 1 66928 | 1 65271 | 1 68736 | 1 69000 | 1 (14 rows)
Relational Algebra Expression:
Relational Algebra Tree:
Sample Database: employee
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Previous: From the following tables, write a SQL query to find those departments where at least two employees work. Return department name, number of employees.
Next: From the following table, write a SQL query to find number of employees and average salary. Group the result set on department id and job name. Return number of employees, average salary, department ID, and job name.
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SQL: Tips of the Day
Delete all Duplicate Rows except for One in MySQL?
SELECT * FROM names; +----+--------+ | id | name | +----+--------+ | 1 | google | | 2 | yahoo | | 3 | msn | | 4 | google | | 5 | google | | 6 | yahoo | +----+--------+
1. If you want to keep the row with the lowest id value:
DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name
2. If you want to keep the row with the highest id value:
DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name
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