Tutorial by Examples: column

To add a new indexed column email to the users table, run the command: rails generate migration AddEmailToUsers email:string:index This will generate the following migration: class AddEmailToUsers < ActiveRecord::Migration[5.0] def change add_column :users, :email, :string add_i...

To remove existing column name from users table, run the command: rails generate migration RemoveNameFromUsers name:string This will generate the following migration: class RemoveNameFromUsers < ActiveRecord::Migration[5.0] def change remove_column :users, :name, :string end end ...

To add a reference to a team to the users table, run this command: $ rails generate migration AddTeamRefToUsers team:references This generates the following migration: class AddTeamRefToUsers < ActiveRecord::Migration[5.0] def change add_reference :users, :team, foreign_key: true ...

To add multiple columns to a table, separate field:type pairs with spaces when using rails generate migration command. The general syntax is: rails generate migration NAME [field[:type][:index] field[:type][:index]] [options] For example, the following will add name, salary and email fields to ...

When a column name matches a reserved keyword, standard SQL requires that you enclose it in double quotation marks: SELECT "ORDER", ID FROM ORDERS Note that it makes the column name case-sensitive. Some DBMSes have proprietary ways of quoting names. For example, SQL Serve...

DT[where, select|update|do, by] syntax is used to work with columns of a data.table. The "where" part is the i argument The "select|update|do" part is the j argument These two arguments are usually passed by position instead of by name. Our example data below is mtcars =...

To do an insert on specific columns (as opposed to all of them) you must specify the columns you want to update. INSERT INTO USERS (FIRST_NAME, LAST_NAME) VALUES ('Stephen', 'Jiang'); This will only work if the columns that you did not list are nullable, identity, timestamp data type or compute...

ALTER TABLE Employees ALTER COLUMN StartingDate DATETIME NOT NULL DEFAULT (GETDATE()) This query will alter the column datatype of StartingDate and change it from simple date to datetime and set default to current date.

There are a couple of ways to delete a column in a DataFrame. import numpy as np import pandas as pd np.random.seed(0) pd.DataFrame(np.random.randn(5, 6), columns=list('ABCDEF')) print(df) # Output: # A B C D E F # 0 -0.895467 0.386902...

# Create a sample DF df = pd.DataFrame(np.random.randn(5, 3), columns=list('ABC')) # Show DF df A B C 0 -0.467542 0.469146 -0.861848 1 -0.823205 -0.167087 -0.759942 2 -1.508202 1.361894 -0.166701 3 0.394143 -0.287349 -0.978102 4 -0.160431 1.054736 -0.785250 ...

df = pd.DataFrame({'old_name_1': [1, 2, 3], 'old_name_2': [5, 6, 7]}) print(df) # Output: # old_name_1 old_name_2 # 0 1 5 # 1 2 6 # 2 3 7 To rename one or more columns, pass the old names and new names as a dictionary: df.r...

df = pd.DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6], 'c': [7, 8, 9]}) To list the column names in a DataFrame: >>> list(df) ['a', 'b', 'c'] This list comprehension method is especially useful when using the debugger: >>> [c for c in df] ['a', 'b', 'c'] This is the long w...

To add a new unique column email to users, run the following command: rails generate migration AddEmailToUsers email:string:uniq This will create the following migration: class AddEmailToUsers < ActiveRecord::Migration[5.0] def change add_column :users, :email, :string add_index...

df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}) print(df) # Output: # A B # 0 1 4 # 1 2 5 # 2 3 6 Directly assign df['C'] = [7, 8, 9] print(df) # Output: # A B C # 0 1 4 7 # 1 2 5 8 # 2 3 6 9 Add a constant column df['C'] = 1 print(df) # Ou...

When you do a groupby you can select either a single column or a list of columns: In [11]: df = pd.DataFrame([[1, 1, 2], [1, 2, 3], [2, 3, 4]], columns=["A", "B", "C"]) In [12]: df Out[12]: A B C 0 1 1 2 1 1 2 3 2 2 3 4 In [13]: g = df.groupby(...

To modify an existing column in Rails with a migration, run the following command: rails g migration change_column_in_table This will create a new migration file in db/migration directory (if it doesn’t exist already), which will contain the file prefixed with timestamp and migration file name w...

If the right-hand side of the assignment is a matrix, then in each iteration the variable is assigned subsequent columns of this matrix. some_matrix = [1, 2, 3; 4, 5, 6]; % 2 by 3 matrix for some_column = some_matrix display(some_column) end (The row vector version is a normal case of thi...

public function up() { $this->dropColumn('post', 'position'); $this->renameColumn('post', 'owner_id', 'user_id'); $this->alterColumn('post', 'updated', $this->timestamp()->notNull()->defaultValue('0000-00-00 00:00:00')); }

public function up() { $this->addColumn('post', 'position', $this->integer()); }

This query will return all COLUMNS and their associated TABLES for a given column name. It is designed to show you what tables (unknown) contain a specified column (known) SELECT c.name AS ColName, t.name AS TableName FROM sys.columns c JOIN sys.tables t ON c.object_id = t.o...