MySQL logical operators enable you to combine multiple conditions in the WHERE clause of a SELECT statement. However, you must understand the concept of operator precedence to use them efficiently.
The logical AND operator compares two Boolean expressions and returns TRUE if both are true. The logical OR operator combines two Boolean terms and returns true if either is true.
Subqueries
A subquery is a query inside a more extensive SQL statement. It can filter, compute aggregate functions, or create a derived table. It is also helpful in reducing the amount of data that has to be processed by the outer query. Subqueries are often used with logical operators such as>, =, IN, and BETWEEN.
In addition, The WHERE clause of a SELECT statement can contain many conditions combined thanks to MySQL logical operators. To use them effectively, you must comprehend the idea of operator precedence.
Subqueries are usually nested within the WHERE clause of the parent query. They can be used with SELECT, INSERT, UPDATE, and DELETE statements, and comparison operators like>, =, and IN. Subqueries can be nested to any depth and may contain multiple queries.
There are two types of subqueries: scalar and non-scalar. Scalar subqueries return one value to the outer query, whereas non-scalar subqueries return more than one value and can be executed independently of the main question. Non-scalar subqueries are more efficient than correlated subqueries run for each outer query row.
Correlated subqueries can be used to retrieve detailed information but are resource intensive. They must be re-evaluated for every set of parameter values, sometimes called “repeating subqueries.” Your query can quickly become resource intensive if it contains many correlated subqueries. Therefore, testing your question under realistic workloads before implementing them in production is essential.
User-defined functions
User-defined functions allow you to encapsulate custom code for a single action and then return that result. These subroutines run within a SQL statement and can take zero or more parameters. They can also produce a default value. UDFs are usually written in a programming language such as C or C++ and then added to MySQL using the CREATE FUNCTION and DROP FUNCTION statements. They can be invoked like a native function such as ABS().
Loadable functions (UDFs) and stored functions use the same interface but are distinguished in that loadable functions can be added to a binary MySQL distribution without modification of the source, while adding a native part requires modifying a source distribution. A SELECT statement can call both.
UDFs are often used to implement parallel processing on large data sets by splitting the data set into partitions, which can then be processed in parallel by different worker processes. When the work is done, the results of each cell must be merged to produce the final result. The UDF process_frame() is called for each section to do this.
UDFs are written in a programming language such as C, C++, or Java and compiled to a shared library that MySQL links into a query. They can be referred to by name in a SELECT statement and accept various arguments from the question. These can be scalar, table, or aggregate functions.
Stored procedures
Stored procedures are pre-compiled SQL statements you can reuse to execute various database operations. You can use stored procedures to perform data manipulation language (DML) commands, including inserts, updates, and deletes. They can also return values from a table and display them to users.
To create a stored procedure, you need to start with the CREATE PROCEDURE command, followed by the name of the process and an optional list of parameters it will accept. The procedure’s body, enclosed in BEGIN and END statements, should contain an SQL statement, such as a SELECT query. You can have multiple SQL statements in one stored procedure, but you should avoid creating one with a complex sequence of tasks. The complexity of these tasks can cause performance problems, especially when the system is called remotely.
Another advantage of stored procedures is that they can be modified without affecting the application that uses them. This makes it easier for developers to update application code and can improve the application’s maintainability over time. It also allows for greater consistency when interacting with the database. For example, when a department shares data between Accounts and HR, the queries can be written in a single stored procedure, avoiding repeated rewrites of the same code.
Query optimization
Query optimization is reordering a query’s operations to improve performance. It can be done by heuristic optimization or cost-based optimization. Heuristic optimization involves analyzing questions and reordering the processes to reduce execution costs. Cost-based optimization is a more systematic approach that compares different evaluation plans and chooses the lowest price. This calculation considers disk access time, CPU execution time, number of operations, size of tuples, and other factors.
The logical OR operator connects multiple conditions in the WHERE clause. It can also be used in the UNION ALL clause to combine results from different tables. This can save a lot of database processing time and memory. However, it is essential to understand the order of precedence for logical operators. Arithmetic operators and comparison operators have higher priority than logical operators.
Another common problem is correlated subqueries, which are costly because they execute row by row. These queries can also be slow to process because they require the database engine to scan all rows, identify duplicate values, and remove duplicate records from the result set. To improve the efficiency of such queries, the database must index a column in both standard tables.
In addition, a SELECT DISTINCT query can significantly impact a database’s performance. To perform this operation, the database must read all rows from both tables and then identify duplicate values. This can be expensive in terms of resources and disk space.
This can be improved by using the GROUP BY clause to group the data based on the segment column, which makes it much faster than retrieving all unique values in each table.
