Article Categories
- All Categories
-
Data Structure
-
Networking
-
RDBMS
-
Operating System
-
Java
-
MS Excel
-
iOS
-
HTML
-
CSS
-
Android
-
Python
-
C Programming
-
C++
-
C#
-
MongoDB
-
MySQL
-
Javascript
-
PHP
-
Economics & Finance
MySQL Articles
Page 59 of 355
Which one is better in MySQL - NULL or empty string?
The choice depends upon the database. In ORACLE database, an empty string is converted to NULL.In MySQL, the usage of an empty string is better as compared to NULL. It is easy to check for an empty string with some boundary conditions, while this cannot be done with NULL. To find NULL, we need to add an extra condition i.e. ‘IS NULL’We can check that the length of NULL is 0 while length of empty string is 1.To check the length of NULL.mysql>SELECT count(NULL);The following is the output of the above query.+-----------------+ | count(NULL) | +-----------------+ ...
Read MoreFetch rows where a field value is less than 5 chars in MySQL?
To fetch rows where a field value is less than 5 chars, you need to use LENGTH() function. The syntax is as follows −SELECT *FROM yourTableName WHERE LENGTH(yourColumnName) < 5;To understand the above syntax, let us create a table. The query to create a table is as follows −mysql> create table fieldLessThan5Chars -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> yourZipCode varchar(10) -> ); Query OK, 0 rows affected (0.52 sec)Now you can insert some records in the table using insert command. The query is as follows −mysql> insert into fieldLessThan5Chars(yourZipCode) values('35801'); Query ...
Read MoreMySQL query to include more than one column in a table that doesn't already exist
You can easily add more than one column that does not exist in a query using multiple AS keywords.Let us first create a table. The query to create a table is as follows −mysql> create table ColumnDoesNotExists -> ( -> UserId int, -> UserName varchar(20) -> ); Query OK, 0 rows affected (0.67 sec)Insert some records in the table using insert command. The query is as follows −mysql> insert into ColumnDoesNotExists(UserId, UserName) values(100, 'Larry'); Query OK, 1 row affected (0.14 sec) mysql> insert into ColumnDoesNotExists(UserId, UserName) values(101, 'Sam'); Query OK, 1 row affected (0.22 sec) mysql> ...
Read MoreAdding a column that doesn't exist in a query?
Add a column that does not exist in a query, with the help of AS keyword. The syntax is as follows −SELECT yourColumnName1, yourColumnName2, ....N, yourValue AS yourColumnName, ....N' FROM yourTableName;To understand the above syntax, let us create a table. The query to create a table is as follows −mysql> create table ColumnDoesNotExists -> ( -> UserId int, -> UserName varchar(20) -> ); Query OK, 0 rows affected (0.67 sec)ExampleInsert some records in the table using insert command. The query is as follows −mysql> insert into ColumnDoesNotExists(UserId, UserName) values(100, 'Larry'); Query OK, ...
Read MoreCheck if a table is empty or not in MySQL using EXISTS
The following is the syntax to check whether a table is empty or not using MySQL EXISTS −SELECT EXISTS(SELECT 1 FROM yourTableName);ExampleFirst, let us create a table. The query to create a table is as follows −mysql> create table ReturnDemo -> ( -> Id int, -> Name varchar(10) -> ); Query OK, 0 rows affected (0.79 sec)Insert some records in the table using insert command. The query is as follows −mysql> insert into ReturnDemo values(100, 'Larry'); Query OK, 1 row affected (0.18 sec) mysql> insert into ReturnDemo values(101, 'Bob'); Query OK, 1 row affected (0.28 sec) ...
Read MoreWhich query is efficient to check if MySQL Table is empty? COUNT(*) vs. LIMIT?
If you use COUNT(*) around the LEAST() then MySQL scans at least one index, therefore avoid LEAST(COUNT(*)) and use LIMIT.Let us first create a table. The query to create a table is as follows −mysql> create table ReturnDemo -> ( -> Id int, -> Name varchar(10) -> ); Query OK, 0 rows affected (0.79 sec)ExampleNow you can insert some records in the table using insert command. The query is as follows −mysql> insert into ReturnDemo values(100, 'Larry'); Query OK, 1 row affected (0.18 sec) mysql> insert into ReturnDemo values(101, 'Bob'); Query OK, 1 row affected (0.28 sec) mysql> insert into ...
Read MoreUsing GROUP BY and MAX on multiple columns in MySQL?
To understand the GROUP BY and MAX on multiple columns, let us first create a table. The query to create a table is as follows −mysql> create table GroupByMaxDemo -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> CategoryId int, -> Value1 int, -> Value2 int -> ); Query OK, 0 rows affected (0.68 sec)ExampleInsert some records in the table using insert command. The query is as follows −mysql> insert into GroupByMaxDemo(CategoryId, Value1, Value2) values(10, 100, 50); Query OK, 1 row affected (0.15 sec) mysql> insert into GroupByMaxDemo(CategoryId, Value1, Value2) values(10, 100, ...
Read MoreHow do I update NULL values in a field in MySQL?
Let us first create a table −mysql> create table OrderDemo -> ( -> OrderId int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> OrderPrice int, -> OrderDatetime datetime -> ); Query OK, 0 rows affected (0.66 sec)ExampleNow you can insert some records in the table using insert command. The query is as follows −mysql> insert into OrderDemo(OrderPrice, OrderDatetime) values(200, '2016-09-12'); Query OK, 1 row affected (0.24 sec) mysql> insert into OrderDemo(OrderPrice, OrderDatetime) values(NULL, '2002-11-18'); Query OK, 1 row affected (0.26 sec) mysql> insert into OrderDemo(OrderPrice, OrderDatetime) values(1000, '2017-12-28'); Query OK, 1 row affected (0.15 sec)Display all records ...
Read MoreHow do I lag columns in MySQL?
To lag a column in MySQL, first, let us create a table. The query to create a table is as follows −mysql> create table LagDemo -> ( -> UserId int, -> UserValue int -> ); Query OK, 0 rows affected (1.74 sec)ExampleInsert some records in the table using insert command. The query is as follows −mysql> insert into LagDemo values(12, 158); Query OK, 1 row affected (0.61 sec) mysql> insert into LagDemo values(18, 756); Query OK, 1 row affected (0.21 sec) mysql> insert into LagDemo values(15, 346); Query OK, 1 row affected (0.25 sec) mysql> insert ...
Read MoreGet a list of Foreign Key constraints in MySQL
Let’s say we have a database “business” with number of tables. If you want to show only foreign key constraints, then use the following query −mysql> select * −> from information_schema.referential_constraints −> where constraint_schema = 'business';The following is the output displaying only foreign key constraints −+--------------------+-------------------+--------------------------+---------------------------+--------------------------+------------------------+--------------+-------------+-------------+-------------------+-----------------------+ | CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | UNIQUE_CONSTRAINT_CATALOG | UNIQUE_CONSTRAINT_SCHEMA | UNIQUE_CONSTRAINT_NAME | MATCH_OPTION | UPDATE_RULE | DELETE_RULE | TABLE_NAME | REFERENCED_TABLE_NAME | +--------------------+-------------------+--------------------------+---------------------------+--------------------------+------------------------+--------------+-------------+-------------+-------------------+-----------------------+ | def | business | ConstChild ...
Read More