Query MySQL using data from table-CodePudding

How can you perform a WHERE query in MySQL using a value found in a specific cell

for example if you have a table full of people and it has the columns first name, middle name, surname perform a SELECT WHERE which finds a single person and then use that persons middle name to find all people with that middle name as their last name.

CodePudding user response：

Do a self join, something like:

select *
from persons p1
left join persons p2 on p2.lastname = p1.middlename
where p1.firstname = 'megaman'

CodePudding user response：

You can use sub queries to find the person of the middle name but inside the WHERE clase of another query this is an example of code

SELECT * FROM CUSTOMERS WHERE ID IN (SELECT ID FROM CUSTOMERS WHERE SALARY > 4500) ;

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.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc.

There are a few rules that subqueries must follow −

Subqueries must be enclosed within parentheses.

A subquery can have only one column in the SELECT clause, unless multiple > >columns are in the main query for the subquery to compare its selected > >columns.

An ORDER BY command cannot be used in a subquery, although the main query >can use an ORDER BY. The GROUP BY command can be used to perform the same >function as the ORDER BY in a subquery.

Subqueries that return more than one row can only be used with multiple >value operators such as the IN operator.

The SELECT list cannot include any references to values that evaluate to a >BLOB, ARRAY, CLOB, or NCLOB.

A subquery cannot be immediately enclosed in a set function.

The BETWEEN operator cannot be used with a subquery. However, the BETWEEN >operator can be used within the subquery.

click here for more information about sub queries.