Learning SQL Command, SQL-Master.net
SQL command dictionary for beginners. Provide both command examples and results.
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COUNT function is used to count a number of rows. If you want to count only unique elements, you can use DISTINCT to exclude duplicated elements.
Example 1: count a number of employees
SELECT COUNT(*) FROM tbEmpTable;
| COUNT(*) |
| 24 |
Example 2: count numbers of employees by each department.
SELECT deptCD, COUNT(*) FROM tbEmpTable GROUP BY deptCD;
| deptCD | COUNT(*) |
| 100 | 3 |
| 200 | 10 |
| 300 | 7 |
| 400 | 4 |
Example 3: count a number of departments
SELECT COUNT(DISTINCT deptCD) FROM tbEmpTable;
| COUNT(DISTINCT deptCD) |
| 4 |
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AVG function to calculate an average value
AVG function is used to calculate an average value from a selected field. Example1: Calculate an average value of employees’ salary. SELECT AVG(salary) FROM tbSalaryTable; AVG(salary) 254000 Example2: Calculate an average value of salary by each department. SELECT deptCD, AVG(salary) FROM tbSalaryTable GROUP BY deptCD; deptCD AVG(salary) 100 150000 200 203090 300 403200 400 305420