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Articles on Trending Technologies
Technical articles with clear explanations and examples
How can I count visitors per page per day using MySQL?
Note: We assume we have created a database named ‘DBNAME’ and a table named ‘tableName’.Let us understand how the count of visitors per day per page can be queried using MySQL. This can be done using the bit group function −QuerySELECT DATE(date) Date, page_id, COUNT(*) colName FROM tableName GROUP BY DATE(date), page_idHere ‘colName’ refers to the ‘visits per day’ column, and ‘tableName’ refers to the table that contains details about visitors.It makes sure that the duplicate values in the table are removed when the above query is run.
Read MoreMaking slow queries fast using composite indexes in MySQL
Let us first see what is Composite Index −A composite index is an index that is used on multiple columns.It is also known as a multiple-column index.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 ...
Read MoreMySQL Composite Index
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 MoreHow to search multiple columns in MySQL?
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 MoreSearching on Two Keys in MySQL
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 MoreUsing User-Defined Variables in MySQL
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 MoreThe Rows Holding the Group-wise Maximum of a Certain Column in MySQL
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 MoreMaximum of Column per Group in MySQL
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 | +--------------+--------------+
Read MoreMajor Contributors to MySQL
Let us see who the major contributors to MySQL are −Although Oracle Corporation and/or its affiliates own all copyrights in the MySQL server and the MySQL manual, we wish to recognize those who have made contributions of one kind or another to the MySQL distribution. Contributors are listed here, in somewhat random order −Gianmassimo Vigazzola − They helped with the initial port to Win32/NT.Per Eric Olsson − They provided for constructive criticism and real testing of the dynamic record format.Irena Pancirov − Helped with the Win32 port with Borland compiler. They also helped with the mysqlshutdown.exe and mysqlwatch.exe.David J. Hughes ...
Read MoreUpgrading MySQL Binary or Package-based Installations on Unix/Linux
Let us understand how MySQL binary and package-based installations can be upgraded in Unix or Linux. This can be done in-place as well as by using a logical upgrade method. Let us understand both these methods in brief −In place upgradeAn in−place upgrade involves shutting down the old MySQL server, replacing the old MySQL binaries or the packages with the new ones.Once this is done, the MySQL server is restarted on the existing data directory.After this, the remaining parts of the existing installation, that require some kind of upgrade, are upgraded.For some Linux platforms, MySQL installation from RPM or Debian ...
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