To get the maximum value from three different columns, use the GREATEST() function.The syntax is as followsSELECT GREATEST(yourColumnName1, yourColumnName2, yourColumnName3) AS anyAliasName FROM yourTableName;To understand the above syntax, let us create a table. The query to create a table is as followsmysql> create table MaxOfThreeColumnsDemo    -> (    -> First int,    -> Second int,    -> Third int    -> ); Query OK, 0 rows affected (0.73 sec)Insert some records in the table using insert command.The query is as followsmysql> insert into MaxOfThreeColumnsDemo values(30, 90, 60); Query OK, 1 row affected (0.16 sec) mysql> insert into MaxOfThreeColumnsDemo values(100, ... Read More

Restart the MySQL Server with the help of restart command.The syntax is as followsRestartLet us first check the MySQL version.The query is as followsSELECT version();Now, implement the above command in order to restart the MySQL Server.The query is as followsmysql> restart; Query OK, 0 rows affected (0.00 sec)Case 1Now, MySQL Server is being restarted. If you try to perform any query during the restart command, you will get an error.The query is as followsmysql> show databases; ERROR 2013 (HY000): Lost connection to MySQL server during queryCase 2If the MySQL restart process is complete, then a new connection id will be ... Read More

You can remove an array element by its index using the following two steps −The first step is as follows −db.yourCollectionName.update({}, {$unset : {"yourArrayFieldName.yourIndexValue" : 1 }});The above syntax will put a null value at the location of ‘yourIndexValue’. After that, you need to pull the null value from array filed to remove from an array element.The second step is as follows −db.yourCollectionName.update({}, {$pull : {"yourArrayFieldName" : null}});To implement the syntax, let us create a collection with the document. The query to create a collection with a document is as follows −> db.removeArrayElementByItsIndexDemo.insertOne({"InstructorName":"David",    "InstructorAge":28, "InstructorSubject":["MongoDB", "MySQL", "Java", "SQL Server", ... Read More

To set auto increment initial value for MySQL table, use ALTER command. The first step would bealter table yourTableName modify yourColumnName int NOT NULL AUTO_INCREMENT PRIMARY KEY, add index(yourColumnName);The second step is as followsalter table yourTableName AUTO_INCREMENT=yourStartingValue;To understand the above syntax, let us create a table. The query to create a table is as followsmysql> create table setAutoIncrementDemo    -> (    -> UserId int,    -> UserName varchar(20)    -> ); Query OK, 0 rows affected (0.75 sec)Now implement the above two steps to set auto increment initial value for MySQL table.Step 1 -The query is as followsmysql> alter ... Read More

Let us see an example and create a table first.mysql> create table Add1ToExistingValue    -> (    -> Value int    -> ); Query OK, 0 rows affected (0.56 sec)Insert some records in the table using insert command.The query is as followsmysql> insert into Add1ToExistingValue values(10); Query OK,  1 row affected (0.12 sec) mysql> insert into Add1ToExistingValue values(13); Query OK,  1 row affected (0.15 sec) mysql> insert into Add1ToExistingValue values(15); Query OK,  1 row affected (0.13 sec) mysql> insert into Add1ToExistingValue values(16); Query OK,  1 row affected (0.14 sec) mysql> insert into Add1ToExistingValue values(20); Query OK,  1 row affected (0.16 sec) mysql> insert into Add1ToExistingValue values(40); Query OK,  1 row affected (0.15 sec) mysql> insert into Add1ToExistingValue values(50); Query OK,  1 row affected (0.11 sec) mysql> insert into Add1ToExistingValue values(55); Query OK,  1 row affected (0.17 sec) mysql> insert into Add1ToExistingValue values(56); Query OK,  1 row affected (0.17 sec)Display all records from the table using select statement.The query is as followsmysql> select *from Add1ToExistingValue;The following is the output+-------+ | Value | +-------+ | 10   ... Read More

Let us work around two cases −Case 1 − The syntax is as follows when the field is present and set to null.db.yourCollectionName.count({yourFieldName: null});Case 1 − The syntax is as follows when the field is not present and not set.db.yourCollectionName.count({yourFieldName: {$exists: false}});To understand both the above syntaxes, let us create a collection with the document. The query to create a collection with a document is as follows −> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Larry", "EmployeeAge":null, "EmployeeSalary":18500}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a995c6cea1f28b7aa07fe") } > db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Bob", "EmployeeAge":21, "EmployeeSalary":23500}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a99836cea1f28b7aa07ff") } > db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Carol", "EmployeeSalary":45500}); { ... Read More

This type of error occurs when number of columns does not match whenever you are inserting records in the destination table. For a demo example, let us create a tablemysql> create table errorDemo    -> (    -> User_Id int NOT NULL AUTO_INCREMENT,    -> User_Name varchar(20),    -> PRIMARY KEY(User_Id)    -> ); Query OK, 0 rows affected (0.47 sec)The error is as followsmysql> insert into errorDemo values('John'); ERROR 1136 (21S01): Column count doesn't match value count at row 1To avoid this type of error, you need to use the following syntaxinsert into yourTableName(yourColumnName1, yourColumnName2, ...N)values(yourValue1, yourValue2, ....N);Insert some ... Read More

To find items that do not have a certain field, use the $exists operator. The syntax is as follows −> db.yourCollectionName.find({"yourItemName":{$exists:false}}).pretty();To understand the syntax, let us create a collection with the document. The query to create a collection with a document is as follows −> db.findDocumentDoNotHaveCertainFields.insertOne({"UserId":101, "UserName":"John", "UserAge":21}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a95fb6cea1f28b7aa07fb") } > db.findDocumentDoNotHaveCertainFields.insertOne({"UserName":"David", "UserAge":22, "UserFavouriteSubject":["C", "Java"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a96116cea1f28b7aa07fc") } > db.findDocumentDoNotHaveCertainFields.insertOne({"UserName":"Bob", "UserAge":24, "UserFavouriteSubject":["MongoDB", "MySQL"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a96306cea1f28b7aa07fd") }Display all documents from a collection with the help of find() method. The ... Read More

To revert rows to default column value, let us first create a demo tablemysql> create table defaultDemo    -> (    -> Id int    -> ); Query OK, 0 rows affected (0.48 sec)Insert some records in the table using insert command.The query is as followsmysql> insert into defaultDemo values(10); Query OK, 1 row affected (0.25 sec) mysql> insert into defaultDemo values(20); Query OK, 1 row affected (0.13 sec) mysql> insert into defaultDemo values(30); Query OK, 1 row affected (0.14 sec) mysql> insert into defaultDemo values(40); Query OK, 1 row affected (0.11 sec) mysql> insert into defaultDemo values(80); Query OK, ... Read More

PyMongo is a Python distribution containing tools for working with MongoDB. To perform regex queries with PyMongo, the syntax is as follows −db.yourCollectionName.find({'yourCollectionName':{'$regex':'^yourWords'}}).pretty();The above syntax will give all those documents that start from a specific word.To understand the syntax, let us create a collection with the document. The query to create a collection with a document is as follows −> db.performRegex.insertOne({"ClientName":"Larry", "ClientFolderInformation":[    "Folder 1", "Folder 2", "Folder 3", "Folder 4", "Folder 5"], "MainFolderLocation":"/MainFolder/Details/ClientFolder" }); {    "acknowledged" : true,    "insertedId" : ObjectId("5c8a8b186cea1f28b7aa07f2") } > db.performRegex.insertOne({"ClientName":"Larry", "ClientFolderInformation":[    "ClientFolder 1", "ClientFolder 2", "ClientFolder 3", "ClientFolder 4", "ClientFolder 5"],   ... Read More