How does distinct work in SQL?

When only one expression is provided in the DISTINCT clause, the query will return the unique values for that expression. When more than one expression is provided in the DISTINCT clause, the query will retrieve unique combinations for the expressions listed. In SQL, the DISTINCT clause doesn’t ignore NULL values.

How does distinct work in SQL with multiple columns?

In SQL multiple fields may also be added with DISTINCT clause. DISTINCT will eliminate those rows where all the selected fields are identical.

Is it good to use distinct in SQL?

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.

Why distinct is bad in SQL?

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.

INTERESTING:  What is the difference between try catch and throws Java?

How set distinct in SQL?

How to use distinct in SQL?

  1. SELECT DISTINCT returns only distinct (different) values.
  2. DISTINCT eliminates duplicate records from the table.
  3. DISTINCT can be used with aggregates: COUNT, AVG, MAX, etc.
  4. DISTINCT operates on a single column.
  5. Multiple columns are not supported for DISTINCT.

Does distinct apply to all columns?

Yes, DISTINCT works on all combinations of column values for all columns in the SELECT clause.

Is distinct faster than group by?

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.

Is select distinct slow?

Very few queries may perform faster in SELECT DISTINCT mode, and very few will perform slower (but not significantly slower) in SELECT DISTINCT mode but for the later case it is likely that the application may need to examine the duplicate cases, which shifts the performance and complexity burden to the application.

Is distinct costly in SQL?

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.

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 distinct or group by better?

Group by is expensive than Distinct since Group by does a sort on the result while distinct avoids it. But if you want to make group by yield the same result as distinct give order by null ..

INTERESTING:  Can SQL Server 2012 run on Windows 2016?

Should I use distinct?

If you’re querying a table that is expected to have repeated values of some field or combination of fields, and you’re reporting a list of the values or combinations of values (and not performing any aggregations on them), then DISTINCT is the most sensible thing to use.

What can I use instead of distinct in SQL?

6 Answers. GROUP BY is intended for aggregate function use; DISTINCT just removes duplicates (based on all column values matching on a per row basis) from visibility. If TABLE2 allows duplicate values associated to TABLE1 records, you have to use either option.

What is difference between unique and distinct in SQL?

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.

How use distinct inner join in SQL Server?

You can use CTE to get the distinct values of the second table, and then join that with the first table. You also need to get the distinct values based on LastName column. You do this with a Row_Number() partitioned by the LastName, and sorted by the FirstName.

How do I use distinct in one column in SQL?

Adding the DISTINCT keyword to a SELECT query causes it to return only unique values for the specified column list so that duplicate rows are removed from the result set. Since DISTINCT operates on all of the fields in SELECT’s column list, it can’t be applied to an individual field that are part of a larger group.

INTERESTING:  Who uses PHP programming language?