Difference Betwixt Where Together With Having Clause Inward Sql?

The master copy departure betwixt WHERE too HAVING clause comes when used together alongside GROUP BY clause, In that illustration WHERE is used to filter rows earlier grouping too HAVING is used to exclude records later on grouping. This is the most of import departure too if you lot retrieve this, it volition assistance you lot write ameliorate SQL queries. This is likewise i of the of import SQL concepts to understand, non simply from an interview perspective but likewise from a day-to-day utilization perspective. I am certain you lot convey used WHERE clause because its i of the most mutual clause inward SQL along alongside SELECT too used to specify filtering standard or condition. You tin fifty-fifty utilization WHERE clause without HAVING or GROUP BY, equally you lot convey seen it many times. 

On the other hand, HAVING tin solely survive used if grouping has been performed using GROUP BY clause inward the SQL query. Another worth noting matter nearly WHERE too HAVING clause is that WHERE clause cannot comprise aggregate component similar COUNT(), SUM(), MAX(), MIN(), etc but  HAVING clause may comprise aggregate functions.

Another worth noting the departure betwixt WHERE too HAVING clause is that WHERE is used to impose filtering standard on a SELECT, UPDATE, DELETE disputation equally good equally unmarried row component too used earlier grouping past times clause but HAVING is ever used later on grouping past times clause.

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Difference betwixt WHERE vs HAVING inward SQL

1) WHERE clause is processed correct later on FROM clause inward the logical gild of query processing, which way it is processed earlier GROUP BY clause spell HAVING clause is executed later on groups are created.

2) If used inward GROUP BY, You tin refer whatever column from a tabular array inward WHERE clause but you lot tin solely utilization columns which are non grouped or aggregated.

3) If you lot utilization HAVING clause without grouping by, it tin likewise refer whatever column but the index volition non survive used equally opposed to WHERE clause. For example, the next convey the same lawsuit set, nonetheless "where" volition utilization the id index too having volition arrive at a tabular array scan

select * from tabular array where id = 1
select * from tabular array having id = 1

4) You tin utilization an aggregate component to filter rows alongside HAVING clause. Because HAVING clause is processed later on the rows convey been grouped, you lot tin refer to an aggregate component inward the logical expression. For example, the next query volition display solely courses which convey to a greater extent than than 10 students :

SELECT Course, COUNT(Course) equally NumOfStudent from Training GROUP BY Course HAVING COUNT(COURSE)> 10

5) Another key departure betwixt WHERE too HAVING clause is that WHERE volition utilization Index too HAVING volition not, for illustration next 2 queries volition create an identical lawsuit but WHERE volition utilization Index too HAVING volition arrive at a tabular array scan

SELECT * FROM Course WHERE Id = 101;
SELECT * FROM Course HAVING Id = 102;


6) Since WHERE clause is evaluated earlier groups are formed, it evaluates for per row. On the other hand, the HAVING clause is evaluated later on groups are formed therefore it evaluates for per group. You tin farther run across the article)

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