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Articles on Trending Technologies
Technical articles with clear explanations and examples
How add unique key to existing table (with non-unique rows)?
You can add unique key to existing table with the help of alter command. The syntax is as follows −ALTER TABLE yourTableName ADD CONSTRAINT yourConstraintName UNIQUE(yourColumnName1, yourColumnName2, ............N);To understand the above concept, let us create a table with some columns. The query to create a table −mysql> create table MovieCollectionDemo −> ( −> MovieId int, −> MovieDirectorName varchar(200), −> NumberOfSongs int unsigned −> ); Query OK, 0 rows affected (0.62 sec)Now you can check the table does not have any unique constraint. The query to check unique constraint is ...
Read MoreGenerating a range of numbers in MySQL?
To generate a range of numbers in MySQL, you can use stored procedure. Firstly, we need to create a table. After that, we will create a stored procedure that generates a range of number from 10 to 1.The following is the query to create a table −mysql> create table GeneratingNumbersDemo −> ( −> Number int −> ); Query OK, 0 rows affected (0.55 sec)Now you can create a stored procedure that stores a range of numbers in the table. The following is the query to create a stored procedure −mysql> delimiter // mysql> CREATE PROCEDURE ...
Read MoreHow to skip first 10 results in MySQL?
To skip first 10 results, use “limit offset”. The syntax is as follows −select *from yourTableName limit 10 offset lastValue;Let us create a table to understand the above syntax. The following is the query to create a table −mysql> create table SkipFirstTenRecords −> ( −> StudentId int, −> StudentName varchar(200) −> ); Query OK, 0 rows affected (0.53 sec)Now you can insert some records in the table with the help of insert command. The query is as follows −mysql> insert into SkipFirstTenRecords values(100, 'John'); Query OK, 1 row affected (0.12 sec) ...
Read MoreMySQL Mass Update with CASE WHEN/ THEN/ ELSE?
The syntax for mass update with CASE WHEN/ THEN/ ELSE is as follows −UPDATE yourTableName set yourColumnName=case when yourColumnName=Value1 then anyUpdatedValue1 when yourColumnName=Value2 then anyUpdatedValue2 when yourColumnName=Value3 then anyUpdatedValue3 when yourColumnName=Value4 then anyUpdatedValue4 else yourColumnName end;To understand the above syntax, let us first create a table. The query to create a table is as follows −mysql> create table CaseUpdateDemo -> ( -> Id int, -> Name varchar(100) -> ); Query OK, 0 rows affected (0.78 sec)Insert some records in the table using insert command. The query is as follows −mysql> insert into CaseUpdateDemo values(1, 'John'); Query OK, 1 row affected ...
Read MoreMySQL always returning the bit values as blank? How to get the original values?
To get the original value, use the following syntax −Syntaxselect yourBitColumnName+0 from yourTableName;The above syntax cast the bit column to an integer. To understand the above concept, let us create a table and check how the returning value is blank. We will also see how to get the original value.The query to create a table.mysql> create table BitDemo -> ( -> hasValidId bit not null -> ); Query OK, 0 rows affected (1.21 sec)Insert some records in the table using insert command. The query to insert records is as follows −mysql> insert into BitDemo values(1); Query OK, 1 row affected ...
Read MoreFetching rows added in last hour with MySQL?
You can use date-sub() and now() function from MySQL to fetch the rows added in last hour.SyntaxThe syntax is as follows −select *from yourTableName where yourDateTimeColumnName create table LastHourRecords -> ( -> Id int, -> Name varchar(100), -> Login datetime -> ); Query OK, 0 rows affected (0.67 sec)Insert records in the form of datetime using insert command. The query to insert record is as follows −mysql> insert into LastHourRecords values(1, 'John', ' 2018-12-19 10:00:00'); Query OK, 1 row affected (0.17 sec) mysql> insert into LastHourRecords values(2, 'Carol', '2018-12-19 10:10:00'); Query OK, 1 row affected (0.15 sec) ...
Read MoreQuery in MySQL for string fields with a specific length?
To query for string fields with a specific length, use the char_length() or length() from MySQL.SyntaxThe syntax is as follows −Case 1 − Use of char_length()This can be used when we are taking length in a number of characters.The syntax −select *from yourTableName where char_length(yourColumnName)=anySpecificLengthValue;Case 2 − Use of length()This can be used when we are taking the length in bytes.The syntax −select *from yourTableName where length(yourColumnName)=anySpecificLengthValue;To understand the above concept, let us first create a table. The query to create a table is as follows −mysql> create table StringWithSpecificLength -> ( -> Id int, -> Name varchar(100), -> FavouriteLanguage ...
Read MoreGet the time difference and convert it to hours in MySQL?
You can achieve with the help of timestampdiff() method from MySQL. The syntax is as follows −SyntaxSELECT ABS(TIMESTAMPDIFF(HOUR, yourColumnName1, yourColumnName2)) as anyVariableName from yourTableName;To understand the above syntax, let us create a table. The query to create a table is as follows −mysql> create table DifferenceInHours -> ( -> StartDateTime datetime, -> EndDateTime datetime -> ); Query OK, 0 rows affected (0.59 sec)Insert some records in the table using insert command. The query is as follows −mysql> insert into DifferenceInHours values('2018-12-20 10:00:00', '2018-12-19 12:00:00'); Query OK, 1 row affected (0.11 sec) mysql> insert into DifferenceInHours ...
Read MoreGet the index of last substring in a given string in MySQL?
To get the index of last substring in a given string, use the char_length() function. First, we need to calculate string length and subtract the last sub string length from the entire length. The difference in length is index of substring.SyntaxThe syntax is as follows −select CHAR_LENGTH(yourColumnName) - LOCATE('yourDelimiter ', REVERSE(yourColumnName))+1 as anyVariableName from yourTableName;To understand the above syntax, let us first create a table. The query to create a table is as follows −mysql> create table SubStringIndexDemo -> ( -> Words varchar(200) -> ); Query OK, 0 rows affected (0.57 sec)Insert some records in the table ...
Read MoreHow to make a primary key start from 1000?
To make a primary key start from 1000, you need to alter your table and set to auto_increment with value 1000. The syntax is as follows −alter table yourTableName auto_increment=1000;To understand the above syntax, let us first create a table. The query to create a table is as follows −mysql> create table PrimaryKey1000Demo -> ( -> ProductId int auto_increment, -> PRIMARY KEY(ProductId) -> ); Query OK, 0 rows affected (0.56 sec)Now here is the query that will update the primary key to start from 1000 −mysql> alter table PrimaryKey1000Demo auto_increment=1000; Query OK, 0 rows affected (0.20 ...
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