How can I get distinct values from a table in SQL?
How to use distinct in SQL?
- SELECT DISTINCT returns only distinct (different) values.
- DISTINCT eliminates duplicate records from the table.
- DISTINCT can be used with aggregates: COUNT, AVG, MAX, etc.
- DISTINCT operates on a single column.
- Multiple columns are not supported for DISTINCT.
How do I select distinct in a table?
The SQL SELECT DISTINCT Statement The SELECT DISTINCT statement is used to return only distinct (different) values. Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.
How to optimize a select distinct x from hugetable?
If you know the values in advance and there is an index on column x (or if each value is likely to appear quickly on a seq scan of the whole table), it is much faster to query each one individually: Proceeding using exists () will do as many index lookups as there are valid values.
When to use SELECT DISTINCT in SQL query?
SELECT DISTINCT is most commonly used SQL clause to get unique rows from tables. But you should avoid using SELECT DISTINCT whenever possible. SELECT DISTINCT works by GROUPing all fields in the query to create distinct results. You can rewrite such queries by adding more unique columns in your column list.
Why is my SELECT DISTINCT query so slow?
Pretending that I’m 100% certain the DISTINCT portion of the query is the reason it runs slowly, I’ve omitted the rest of the query to avoid confusion, since it is the distinct portion’s slowness that I’m primarily concerned with (distinct is always a source of slowness). The table in question has 2.5 million rows of data.
How to speed up SELECT DISTINCT query in Postgres?
If you put an index on the column (s) selected by the query, the database may be able to read them out in index order and save the sort step. A lot will depend on the details of the query and the tables involved– your saying you “know the problem is with the DISTINCT” really limits the scope of available answers.