MySQL Subqueries

A subquery may occur in: ...
In MySQL subquery can be nested inside a SELECT, INSERT, UPDATE, DELETE, SET, or DO statement or inside another subquery.
A subquery is usually added within the WHERE Clause of another SQL SELECT statement.
You can use the comparison operators, such as >, <, or =.

What is a MySQL subquery?
Does MySQL support subquery?
Are subqueries bad?
How optimize subquery MySQL?
How do I write a complex query in MySQL?
What is join query in MySQL?
Which is not a valid comment style in MySQL?
Is not supported by MySQL?
Which key is used to link two tables in MySQL?
Which is better joins or subqueries?
Why use subqueries instead of joins?
Do subqueries hurt performance?

What is a MySQL subquery?

A subquery in MySQL is a query, which is nested into another SQL query and embedded with SELECT, INSERT, UPDATE or DELETE statement along with the various operators. We can also nest the subquery with another subquery.

Does MySQL support subquery?

MySQL supports three types of subqueries, scalar, row and table subqueries. ... Table subqueries can return multiple rows as well as columns. Subqueries can also be used in INSERT, UPDATE and DELETE queries.

Are subqueries bad?

No, the presence of subqueries does not necessarily mean a database schema is poorly designed. Correlated subqueries should be used sparingly (i.e. when an inner condition refers to an outer clause). Other than that, subqueries are often a useful and a natural way of solving a problem.

How optimize subquery MySQL?

13.2. 10.10 Optimizing Subqueries

Use subquery clauses that affect the number or order of the rows in the subquery. ...
Replace a join with a subquery. ...
Some subqueries can be transformed to joins for compatibility with older versions of MySQL that do not support subqueries. ...
Move clauses from outside to inside the subquery.

How do I write a complex query in MySQL?

Example of how to build complex MySQL queries in dbForge Studio for MySQL

Enabling Query Builder. ...
Adding tables to the query. ...
Creating subqueries. ...
Creating JOINs between the tables. ...
Creating GROUP BY or ORDER BY clauses. ...
Viewing and executing the query. ...
Analyzing the result.

What is join query in MySQL?

MySQL JOINS are used with SELECT statement. It is used to retrieve data from multiple tables. It is performed whenever you need to fetch records from two or more tables. ... MySQL INNER JOIN (or sometimes called simple join) MySQL LEFT OUTER JOIN (or sometimes called LEFT JOIN)

Which is not a valid comment style in MySQL?

When parsing SQL code, MySQL ignores the comments part. ... The double dash-comment style requires at least whitespace or control character (space, tab, newline, etc) after the second dash. SELECT * FROM users; -- This is a comment. Note that standard SQL does not require a whitespace after the second dash.

Is not supported by MySQL?

The correct answer to the question “Which of the following is not supported by MySQL” is option (D). Stored Procedures. Because all the other options like Temporary Tables, Table Joining, and Regular Expression Matching is supported by MySQL.

Which key is used to link two tables in MySQL?

Foreign Key is only the Key which can link two tables.

Which is better joins or subqueries?

The advantage of a join includes that it executes faster. The retrieval time of the query using joins almost always will be faster than that of a subquery. By using joins, you can maximize the calculation burden on the database i.e., instead of multiple queries using one join query.

Why use subqueries instead of joins?

Subqueries can be used to return either a scalar (single) value or a row set; whereas, joins are used to return rows. A common use for a subquery may be to calculate a summary value for use in a query. For instance we can use a subquery to help us obtain all products have a greater than average product price.

Do subqueries hurt performance?

The things you read on the internet, for example, “don't use a sub-query because that hurts performance.”