Is SQL JOIN bad?
Table of Contents
Is SQL JOIN bad?
The problem is joins are relatively slow, especially over very large data sets, and if they are slow your website is slow. It takes a long time to get all those separate bits of information off disk and put them all together again.
Is SQL JOIN necessary?
Using the SQL JOIN clause is necessary if you want to query multiple tables. It’s the nature of relational databases in general – they consist of data that’s usually saved in multiple tables; in turn, these form a database. Once you get familiar with basic SQL queries, it’s wise to start learning the JOIN clause.
Is inner join bad?
Doing an INNER join is not so bad, it is what databases are made for. The only time it is bad is when you are doing it on a table or column that is inadequately indexed.
Why LEFT join giving more rows?
7 Answers. LEFT JOIN can return multiple copies of the data from table1, if the foreign key for a row in table 1 is referenced by multiple rows in table2. GROUP BY aggregates rows based on a field, so this will collapse all the table1 duplicates into one row.
What does left outer join do in SQL?
Left Outer Join returns all the rows from the table on the left and columns of the table on the right is null padded. Left Outer Join retrieves all the rows from both the tables that satisfy the join condition along with the unmatched rows of the left table.
What are SQL subqueries?
A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within the WHERE clause. A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved. A subquery cannot be immediately enclosed in a set function.
Can LEFT join cause duplicates?
Join duplications For example, if you have a left table with 10 rows, you are guaranteed to have at least 10 rows after the join, but you may also have 20 or 100 depending on what you are joining to. This happens twice, once for each “Tissues” row in the left table, yielding two duplicated rows.