Best answer: How do you select unique combinations of two columns in SQL?

How do I get unique two column combinations in SQL?

To select distinct combinations from two columns, you can use CASE statement. Let us create a table with some columns.

How do you check if combination of columns is unique in SQL?

Use the HAVING clause to easily identify duplicates. If that query returns 0 rows, the three columns are (currently) unique. If that query returns 1 or more rows, you’ll know which values are duplicated.

Can we apply distinct on multiple columns?

Answer. Yes, the DISTINCT clause can be applied to any valid SELECT query. It is important to note that DISTINCT will filter out all rows that are not unique in terms of all selected columns.

How do I get unique columns in SQL?

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.

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How do I count multiple columns in SQL?

There are several things you can count with COUNT() function:

  1. count(*) : rows.
  2. count(col1) : rows where col1 is not null.
  3. count(col2) : rows where col2 is not null.
  4. count(distinct col1) : distinct col1 values.
  5. count(distinct col2) : distinct col2 values.

Can you group by multiple columns in SQL?

SQL GROUP BY multiple columns is the technique using which we can retrieve the summarized result set from the database using the SQL query that involves grouping of column values done by considering more than one column as grouping criteria.

What is a unique key in SQL?

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.

Are composite keys unique?

A composite unique key is a unique key made up of a combination of columns. Oracle creates an index on the columns of a unique key, so a composite unique key can contain a maximum of 16 columns. … Any row that contains nulls in all key columns automatically satisfies the constraint.

How do I SELECT multiple columns based on condition in SQL?

CREATE TYPE foo AS (new1 text, new2 int); Then you can select multiple columns based on a single CASE expression and unnest the record in the same step: SELECT tbl_id, (CASE WHEN TRUE THEN (col1::text, col2::int)::foo ELSE (col2::text, col3::int)::foo END).

How do I SELECT one column as distinct?

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.

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How does SELECT distinct work?

A SELECT DISTINCT statement first builds our overall result set with all records, i.e including duplicate values based on FROM, JOIN, WHERE, HAVING, etc statements. Next, it sorts the result set based on the column_name or field with which DISTINCT has been used.

How many tables can be join in SQL query?

Theoretically, there is no upper limit on the number of tables that can be joined using a SELECT statement. (One join condition always combines two tables!) However, the Database Engine has an implementation restriction: the maximum number of tables that can be joined in a SELECT statement is 64.