How do you avoid duplicates when joining two tables?

How do you avoid duplicates when joining two tables?

The keyword DISTINCT is used to eliminate duplicate rows from a query result: SELECT DISTINCT FROM A JOIN B ON However, you can sometimes (possibly even ‘often’, but not always) avoid the need for it if the tables are organized correctly and you are joining correctly.

How do I prevent duplicate rows in join?

1. ALWAYS put the join predicates in the join.
2. ALSO, sometimes you want to join to the same table more than once, with DIFFERENT predicates for each join.

How do you prevent duplicates in Oracle table?

Best Answer

1. Create a VIEW for your table ( SELECT * FROM MyTable);
2. Create an INSTEAD-OF INSERT trigger ON the view that does the insert.
3. Create an UNIQUE INDEX “MyUniqueIndexName” on the columns you need to avoid duplicates.
4. Use the following EXCEPTION section on the TRIGGER:

How can we delete duplicate rows from one column in Oracle?

Delete duplicate rows by identifying their column. After “SQL'” enter “delete from names a where rowid > (select min(rowid) from names b where b.name=a.name and b. age=a. age);” to delete the duplicate records.

Can primary key accept duplicate values?

A primary key is a column of table which uniquely identifies each tuple (row) in that table. Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values.

How to do left join without duplicate rows from left table?

The OUTER APPLY selects a single row (or none) that matches each row from the left table. The GROUP BY performs the entire join, but then collapses the final result rows on the provided columns.

Are there duplicate records on the left side of the table?

The left-side items WILL be duplicated for each record found in the right-side items. Your UI code has to filter out the duplicated items in the left side of the query. This is a display issue, not a query issue. +5 even if this is possible to generate such of result! Please Sign up or sign in to vote.

Can a SQL query generate a result with duplicates?

Please Sign up or sign in to vote. You cannot generate a result like that in an SQL query. The left-side items WILL be duplicated for each record found in the right-side items. Your UI code has to filter out the duplicated items in the left side of the query.

Can a left join be left empty in SQL?

In case they have no salary that field can be left empty. So far a left join is all we need. But I only want one row per user. Due to some defects there can be several salaries for one user (see table salary). I only want one row per user which can be selected randomly (or top 1).