## MySQL Composite Index

Updated on 09-Mar-2021 13:22:31
A composite index is an index that is used on multiple columns. It is also known as a multiplecolumn index.FeaturesLet us see the features −MySQL allows the user to create a composite index which can consist of up to 16 columns.The query optimizer uses the composite indexes for queries which will test all columns in the index.It can also be used for queries which will test the first columns, the first two columns, and so on.If the columns are specified in the right order in the index definition, a single composite index can be used that would speed up certain ... Read More

## How to search multiple columns in MySQL?

Updated on 09-Mar-2021 13:20:27
Let us understand how to search multiple columns in MySQL −Note: We assume we have created a database named ‘DBNAME’ and a table named ‘tableName’.The ‘AND’ and ‘OR’ operators can be used, depending on what the user wants the search to return.Let us see that with the help of an example −ExampleSELECT colName FROM tableName WHERE my_col LIKE %$param1% AND another_col LIKE %$param2%;In the above example, the ‘AND’ operator is used.This means that both the clause has to match a record for the result to be returned.QuerySELECT colName FROM tableName WHERE my_col LIKE %$param1% OR another_col LIKE %$param2%;In the above ... Read More

## Searching on Two Keys in MySQL

Updated on 09-Mar-2021 13:18:53
Let us understand how to search on two keys in MySQLSearching on two keys can be achieved using the ‘OR’ with the help of single key which is well optimized or using ‘AND’ which is well optimized. Let us see how searching on two different keys can be done combining it with ‘OR’ operation −SELECT field1_index, field2_index FROM tableName WHERE field1_index = '1' OR field2_index = '1'This is an optimized version of the query. It can also be done efficiently using the ‘UNION’ that combines the output of two separate ‘SELECT’ statements. Every ‘SELECT’ statement searches only for one key ... Read More

## Using Foreign Keys in MySQL

Updated on 09-Mar-2021 13:16:13
Let us understand how foreign keys can be used in MySQL −InnoDB tables support checking foreign key constraints. A foreign key constraint isn’t required just to join two tables. It can be used while defining a column that needs to be used, for storage engines apart from InnoDB. The REFERENCES tableName(colName) has no real effect and serves as a comment to the user that the column currently being defined is intended to refer to column in a different table.MySQL doesn’t do any check to ensure that ‘colName’ actually exists in the ‘tableName’ or whethr ‘tableName’ itself truly exists.In the parent ... Read More

## Using User-Defined Variables in MySQL

Updated on 09-Mar-2021 13:14:48
Let us understand what user variables are and how they can be used in MySQL. We will also see the rules −User variables are written as @var_name. Here, the ‘var_name’ refers to variable name, which consists of alphanumeric characters, ., _, and \$.A user variable name can contain other characters if they are quoted as a string or identifier.User-defined variables are session specific.A user variable which is defined by one client can’t be seen or used by other clients.But the only exception is that if a user has access to the Performance Schema user_variables_by_thread table, then that user can see ... Read More

## The Rows Holding the Group-wise Maximum of a Certain Column in MySQL

Updated on 09-Mar-2021 13:13:27
Let us understand how to find the rows that hold the group wise maximum of a specific column in MySQL −The syntax to find the rows that hold the group-wise maximum of a specific column in MySQL is as follows −SELECT colName1, colName2, colName3 FROM tableName s1 WHERE colName3=(SELECT MAX(s2. colName3) FROM tableName s2 WHERE s1. colName1= s2. colName1) ORDER BY colName1;Let’s say we have the following PRODUCT Table −+---------+----------+--------+ | Article | Warehouse| Price  | +---------+----------+--------+ | 1       | North    | 255.50 | | 1       | North    | 256.05 | | ... Read More

## Maximum of Column per Group in MySQL

Updated on 09-Mar-2021 13:11:12
Let us understand how to find the maximum of a column per group in MySQL −SELECT colName1, MAX(colName2) FROM tableName GROUP BY colName1 ORDER BY colName1;We will now see a live example. Let’s say we have a table PRODUCT −+---------+--------+ | Article | Price  | +---------+--------+ | 1       | 255.50 | | 1       | 256.05 | | 2       | 90.50  | | 3       | 120.50 | | 3       | 123.10 | | 3       | 122.10 | +---------+--------+Following is the query to get the maximum of column per group −QuerySELECT Article, MAX(Price) AS MaxPrice FROM Product GROUP BY Article ORDER BY Article;Output+--------------+--------------+ | Article      | MaxPrice | +--------------+--------------+ | 0001         | 256.05 | | 0002         | 90.50 | | 0003 | 123.10 | +--------------+--------------+

## The Row Holding the Maximum of a Certain Column in MySQL

Updated on 08-Mar-2021 12:29:09
Let us understand how to find the row that holds the maximum of a specific column in MySQL −Note: We assume we have created a database named ‘DBNAME’ and a table named ‘tableName’.Let us see how to fetch the row that holds the maximum value of a specific column using a MySQL query −This can be done using the subquery. Here, we are fetching the maximum value of colName3 −QuerySELECT colName1, colName2, colName3 FROM tableName WHERE colName3=(SELECT MAX(colName3) FROM tableName);Output+--------------+--------------+--------------+ | colName1 | colName2 | colName3 | +--------------+--------------+--------------+ ... Read More

## Getting Minimum and Maximum Value in MySQL

Updated on 09-Mar-2021 06:51:46
We need to use the MAX(columnName) to find the Maximum value in a column, whereas use the MIN(columnName) to find the Maximum value in a column.Let’s say following is the syntax to find the highest and lowest value in a specific column −mysql> SELECT @min_val:=MIN(columnName), @max_val:=MAX(columnName) FROM tableName; mysql> SELECT * FROM tableName WHERE columnName=@min_val OR columnName=@max_val;Note: Let’s say we have a database named ‘StudentsRecords’ and a table named ‘STUDENT.Following is our table −StudentIdStudentMarksS00190S00297S00372We will now write the query −Querymysql> SELECT @min_val:=MIN(StudentMarks), @max_val:=MAX(StudentMarks) FROM STUDENT; mysql> SELECT * FROM STUDENT WHERE StudentMarks =@min_val OR StudentMarks =@max_val;Output+---------------------+ | StudentMarks   ... Read More

## Find the Maximum Value in a Column in MySQL

Updated on 08-Mar-2021 12:26:03
We need to use the MAX(columnName) to find the Maximum value in a column. But, at first, we will understand about database and tables in MySQL.Before installing MySQL, it is important to determine which version and which distribution format (it could be a binary file or from source files) should be used. If the database was newly created, it is obvious that there would be no tables in it.One of the most important part is to decide the structure of the database, the tables that would be needed, the columns in every table and the relationship between these tables. Once ... Read More