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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
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
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
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
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
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 | +--------------+--------------+
Files is collection of records (or) it is a place on hard disk, where data is stored permanently. By using C commands, we access the files in different ways.Need of files in C languageEntire data is lost when the program terminates and storing in a file will preserve your data even if the program terminates.If you want to enter a large amount of data, normally, it takes a lot of time to enter them all.If you have a file containing all the data, you can easily access the contents of the file by using few commands in C.You can easily ... Read More
A union is called as a memory location, which is shared by several variables of different data types.SyntaxThe syntax is as follows −union uniontag{ datatype member 1; datatype member 2; ---- ---- datatype member n; };For example, union sample{ int a; float b; char c; };Declaration of union variableGiven below are the respective declarations of union variable −Union sample{ int a; float b; char c; }s;Union{ int a; float b; char c; }s;Union sample{ int a; float b; char c; }; union sample ... Read More
If the structure is nested inside a union, it is called as a union of structures. There is a possibility to create a union inside a structure in C programming language.ExampleFollowing is the C program for union of structures −#include struct x { int a; float b; }; union z{ struct x s; }; main ( ){ union z u; u.s.a = 10; u.s.b = 30.5; printf("a=%d", u.s.a); printf("b=%f", u.s.b); getch ( ); }OutputWhen the above program is executed, it produces the following result −a= 10 b = 30.5ExampleGiven below is ... Read More
Structure is a collection of different datatype variables, grouped together under a single name.Features of structureThe features of structure are explained below −It is possible to copy the contents of all structure elements of different datatypes to another structure variable of its type by using an assignment operator.For handling complex datatypes, it is better to create a structure within an another structure, which is called as the nested structures.It is possible to pass an entire structure, individual elements of a structure and an address of structure to a function.It is also possible to create the structure pointers.Declaration of structuresThe general ... Read More