Category: Zagon

MariaDB – Add calculated field that shows day name

ALTER TABLE tablename 
ADD COLUMN dayname VARCHAR(20) GENERATED ALWAYS AS (DAYNAME(startdate)) 
STORED;

This query will add a new column called dayname to the tablename table. The column will be of type VARCHAR with a maximum length of 20 characters. The GENERATED ALWAYS clause tells MariaDB to calculate the value of the column each time a row is inserted or updated. The AS keyword specifies the expression used to calculate the value, in this case the DAYNAME function applied to the startdate column. Finally, the STORED keyword tells MariaDB to store the calculated value in the table so that it can be retrieved more efficiently.

Note that the GENERATED ALWAYS and STORED clauses require MariaDB version 5.7.6 or later. If you are using an earlier version of MariaDB, you can still add a calculated field using a trigger or a view.

SQL Server Express – starting to work with Geometry Functions in SQL Server

Firstly I created a database.

This is essentially me trying to implement in SQL Server what I had written about in Postgres here – Using ST_Within in Postgres

Now I create the starting tables.

CREATE TABLE [dbo].[t00001fields](
	[pkid] [int] IDENTITY(1,1) NOT NULL,
	[fieldname] [varchar](50) NULL,
	[geom] [geometry] NULL

Then

CREATE TABLE [dbo].[t00002plots](
	[pkid] [int] IDENTITY(1,1) NOT NULL,
	[plotname] [varchar](50) NULL,
	[geom] [geometry] NULL

Now a magic junction table that will identify what plot is within which field when it is run

CREATE VIEW v001FieldPlotJunction AS
SELECT t00002plots.pkid as Plotspkid,t00001fields.pkid as Fieldspkid
FROM
t00002plots,
t00001fields
WHERE
t00002plots.GEOM.STWithin(t00001fields.GEOM) = 1;

and here’s an example of it running on local in SQL Express

And this is what I am seeing when viewed through QGIS = many of the fields are smaller than the plots and so although 1 2 look like they should be in the view because the plot is not WITHIN the field it does not show which is the correct response in this case.

Batch File to split CSV into smaller parts

So I am using CHATGPT regularly now and this is a good example of where it can prove very useful. The bottom bat iteration was obtained from StackOverflow and successfully split a large file into chunks of 2,500. I then realised that each sub file needed to have headers in it to assist in importing the information into a database. I asked CHATGPT to alter the starting file to ensure that headers were included subsequently.

It did it no problem..

@echo off

setlocal ENABLEDELAYEDEXPANSION
REM Edit this value to change the name of the file that needs splitting. Include the extension.

SET BFN=C:\csv\target.csv

REM Edit this value to change the number of lines per file.
SET LPF=2500

REM Edit this value to change the name of each short file. It will be followed by a number indicating where it is in the list.
SET SFN=SplitFile

REM Do not change beyond this line.

SET SFX=%BFN:~-3%
SET /A LineNum=0
SET /A FileNum=1
set "header="
for /f "tokens=* usebackq" %%a in ("%BFN%") do (

if not defined header (
set "header=%%a"
echo !header! > %SFN%!FileNum!.%SFX%
) else (
SET /A LineNum+=1
echo %%a >> %SFN%!FileNum!.%SFX%
if !LineNum! EQU !LPF! (
SET /A LineNum=0
SET /A FileNum+=1
echo !header! > %SFN%!FileNum!.%SFX%
)
)

)

endlocal

Pause

This is the original obtained from Stack Overflow..

@echo off

setlocal ENABLEDELAYEDEXPANSION

REM Edit this value to change the name of the file that needs splitting. Include the extension.
SET BFN=C:\csv\target.csv

REM Edit this value to change the number of lines per file.
SET LPF=2500

REM Edit this value to change the name of each short file. It will be followed by a number indicating where it is in the list.
SET SFN=SplitFile

REM Do not change beyond this line.

SET SFX=%BFN:~-3%
SET /A LineNum=0
SET /A FileNum=1
For /F "delims==" %%l in (%BFN%) Do (
SET /A LineNum+=1
echo %%l >> %SFN%!FileNum!.%SFX%
if !LineNum! EQU !LPF! (
SET /A LineNum=0
SET /A FileNum+=1

)

)

endlocal

Pause

MS Azure – TSQL a simple example using a Common Table Expression (CTE)

A common table expression (CTE) is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE is similar to a derived table in that it is not stored as an object and exists only for the duration of the query.

I’m finding them useful as you can use them within a view to contain all the references required for that particular view. This I feel makes things more maintainable.

An example of a simple common table expression (CTE) in T-SQL:

WITH CTE_EmployeeSalaries AS
(
    SELECT EmployeeID, Salary
    FROM Employees
    WHERE Salary > 50000
)
SELECT *
FROM CTE_EmployeeSalaries;

This CTE defines a virtual table named CTE_EmployeeSalaries that contains all employees with a salary greater than 50,000. The CTE is then used to select all rows from this virtual table.

Common table expressions (CTEs) have several advantages:

1.Improved readability: can make complex queries easier to read and understand by breaking them up into smaller, more manageable pieces.
2.Improved maintainability: Because CTEs are self-contained, they can be easily modified or replaced without affecting the rest of the query.
3.Improved performance: In some cases can improve query performance by reducing the need for subqueries.
4.Recursive queries: CTEs can be used to perform recursive queries, which are useful for querying hierarchical data such as an organizational chart or a bill of materials.
5.Temporary results: can be used to store intermediate results that are used multiple times within a single query. This can be more efficient than repeatedly calculating the same intermediate results.
6.Better alternative to view: CTEs can be used as an alternative to views, which can be difficult to modify and are not always optimized for performance.
7.Reusable: CTEs can be defined once and used multiple times within a single query or in multiple queries within the same session.

MS Azure TSQL writing comments

In TSQL, comments can be added to code in two ways:

1.Single-line comments: These start with two hyphens (–) and continue until the end of the line. For example:

-- This is a single-line comment

2.Multi-line comments: These start with a forward slash and an asterisk (/) and continue until the closing asterisk and forward slash (/). For example:

/*
This is a multi-line comment. It can span
multiple lines and is often used to provide
detailed explanations or to comment out large
blocks of code.
*/

When writing comments, it’s important to keep them clear and concise, and to use them to explain why something is being done rather than how it is being done. This helps to make your code more readable and easier to understand for other developers who may be reading it.