group by vs distinct

DISTINCT is used to filter unique records out of all records in the table. It removes the duplicate rows. SELECT DISTINCT will always be the same, or faster than a GROUP BY.

What is the difference between GROUP BY and distinct?

GROUP BY lets you use aggregate functions, like AVG , MAX , MIN , SUM , and COUNT . On the other hand DISTINCT just removes duplicates. This will give you one row per department, containing the department name and the sum of all of the amount values in all rows for that department.

What are the main differences between select distinct and GROUP BY?

Distinct is used to find unique/distinct records where as a group by is used to group a selected set of rows into summary rows by one or more columns or an expression. The functional difference is thus obvious.

Is distinct an expensive operation?

In a table with million records, SQL Count Distinct might cause performance issues because a distinct count operator is a costly operator in the actual execution plan.

Should I use distinct?

The distinct keyword is used in conjunction with select keyword. It is helpful when there is a need of avoiding duplicate values present in any specific columns/table. When we use distinct keyword only the unique values are fetched.

What can we use instead of GROUP BY?

SQL Sub-query as a GROUP BY and HAVING Alternative

You can use a sub-query to remove the GROUP BY from the query which is using SUM aggregate function. There are many types of subqueries in Hive, but, you can use correlated subquery to calculate sum part.

Can we use distinct and GROUP BY Together in Oracle?

We can use GROUP BY without specifying any aggregate functions in the SELECT list. However, the same result is usually produced by specifying DISTINCT instead of using GROUP BY.

Is GROUP BY faster than distinct postgresql?

From experiments, I founded that the GROUP BY is 10+ times faster than DISTINCT. They are different. So what I learned is: GROUP-BY is anyway not worse than DISTINCT, and it is better sometimes.

What is the difference between distinct and unique?

Unique and Distinct are two SQL constraints. The main difference between Unique and Distinct in SQL is that Unique helps to ensure that all the values in a column are different while Distinct helps to remove all the duplicate records when retrieving the records from a table.

What is difference between GROUP BY and order by?

1. Group by statement is used to group the rows that have the same value. Whereas Order by statement sort the result-set either in ascending or in descending order.

Which is better distinct or GROUP BY in Oracle?

DISTINCT implies you want a distinct set of columns. However, GROUP BY implies you want to compute some sort of aggregate value which you are not.

Why is Count distinct so slow?

It’s slow because the database is iterating over all the logs and all the dashboards, then joining them, then sorting them, all before getting down to real work of grouping and aggregating.

Why is distinct slow?

Why DISTINCT queries are slow on PostgreSQL

Why are DISTINCT queries slow on PostgreSQL when they seem to ask an “easy” question? It turns out that PostgreSQL currently lacks the ability to efficiently pull a list of unique values from an ordered index.

Is SELECT distinct bad practice?

As a general rule, SELECT DISTINCT incurs a fair amount of overhead for the query. Hence, you should avoid it or use it sparingly. The idea of generating duplicate rows using JOIN just to remove them with SELECT DISTINCT is rather reminiscent of Sisyphus pushing a rock up a hill, only to have it roll back down again.

Is distinct bad?

So, is SQL DISTINCT good or bad in removing duplicates in results? The results say that it’s good. It’s not better or worse than GROUP BY because the plans are the same. But it’s a good habit to check the execution plan.

Does GROUP BY eliminate duplicates?

SQL delete duplicate Rows using Group By and having clause

In this method, we use the SQL GROUP BY clause to identify the duplicate rows. The Group By clause groups data as per the defined columns and we can use the COUNT function to check the occurrence of a row.

Why distinct is bad in SQL?

Summary. DISTINCT makes a query return unique rows only, and is often used injudiciously to suppress duplicate rows being returned by a bad query. Don’t use DISTINCT to cover up errors.

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