select subset of data with labels in specific column balanced in TSQL

I have about 4M records in a table. Each record in this table is distinct. One of the columns 'labels' has two distinct values L1 and L2. I want to randomly select 100000 records such that 50% of this data has label L1 and the rest has L2.

I have about 4M records in a table. Each record in this table is distinct. One of the columns 'labels' has two distinct values L1 and L2. I want to randomly select 100000 records such that 50% of this data has label L1 and the rest has L2.

Eg : If the following table is the mastertable and has about 10 rows

s_key  C1    C2    labels
s1     0.2   0.1    L1
s2     0.3   0.3    L1
s3     0.25  0.32   L2
s4     1.4   0.5    L1
s5     2.3   9.0    L1
s6     1.2   0.8    L1
s7     0.8   0.4    L1
s8     2.6   9.5    L2
s9     3.7   7.1    L2
s10    2.7   8.3    L2

I want to randomly select 4 records from the above table such that 50 % that is 2 records have labels L1 and 2 records have labels L2 and looks like the following

s_key  C1    C2    labels
s1     0.2   0.1    L1
s2     0.3   0.3    L1
s3     0.25  0.32   L2
s10    2.7   8.3    L2

How can I do this in Tsql/Sql? Appreciate any input.

What are the differences between Standard SQL and Transact-SQL?

What are the differences between Standard SQL and Transact-SQL?

In this article, we'll explain syntax differences between standard SQL and the Transact-SQL language dedicated to interacting with the SQL

#1 Names of Database Objects

In relational database systems, we name tables, views, and columns, but sometimes we need to use the same name as a keyword or use special characters. In standard SQL, you can place this kind of name in quotation marks (""), but in T-SQL, you can also place it in brackets ([]). Look at these examples for the name of a table in T-SQL:

CREATE TABLE dbo.test.“first name” ( Id INT, Name VARCHAR(100));
CREATE TABLE dbo.test.[first name]  ( Id INT, Name VARCHAR(100));

Only the first delimiter (the quotation marks) for the special name is also part of the SQL standard.

What Is Different in a SELECT Statement?#2 Returning Values

The SQL standard does not have a syntax for a query returning values or values coming from expressions without referring to any columns of a table, but MS SQL Server does allow for this type of expression. How? You can use a SELECT statement alone with an expression or with other values not coming from columns of the table. In T-SQL, it looks like the example below:

SELECT 12/6 ;

In this expression, we don’t need a table to evaluate 12 divided by 6, therefore, the FROM statement and the name of the table can be omitted.

#3 Limiting Records in a Result Set

In the SQL standard, you can limit the number of records in the results by using the syntax illustrated below:


T-SQL implements this syntax in a different way. The example below shows the MS SQL Server syntax:


As you notice, this uses an ORDER BY clause. Another way to select rows, but without ORDER BY, is by using the TOP clause in T-SQL:

#4 Automatically Generating Values

The SQL standard enables you to create columns with automatically generated values. The syntax to do this is shown below:


In T-SQL we can also automatically generate values, but in this way:

#5 Math Functions

Several common mathematical functions are part of the SQL standard. One of these math functions is CEIL(x), which we don’t find in T-SQL. Instead, T-SQL provides the following non-standard functions: SIGN(x), ROUND(x,[,d]) to round decimal value x to the number of decimal positions, TRUNC(x) for truncating to given number of decimal places, LOG(x) to return the natural logarithm for a value x, and RANDOM() to generate random numbers. The highest or lowest number in a list in the SQL standard is returned by MAX(list) and MIN(list) functions, but in Transact-SQL, you use the GREATEST(list) and LEAST(list) functions.

T-SQL function ROUND:


#6 Aggregate Functions

We find another syntax difference with the aggregate functions. The functions COUNT, SUM, and AVG all take an argument related to a count. T-SQL allows the use of DISTINCT before these argument values so that rows are counted only if the values are different from other rows. The SQL standard doesn't allow for the use of DISTINCT in these functions.

Standard SQL:



But in T-SQL we don’t find a population covariance function: COVAR_POP(x,y), which is defined in the SQL standard.

#7 Retrieving Parts of Dates and Times

Most relational database systems deliver many functions to operate on dates and times.

In standard SQL, the EXTRACT(YEAR FROM x) function and similar functions to select parts of dates are different from the T-SQL functions like YEAR(x) or DATEPART(year, x).

There is also a difference in getting the current date and time. Standard SQL allows you to get the current date with the CURRENT_DATE function, but in MS SQL Server, there is not a similar function, so we have to use the GETDATE function as an argument in the CAST function to convert to a DATE data type.

#8 Operating on Strings

Using functions to operate on strings is also different between the SQL standard and T-SQL. The main difference is found in removing trailing and leading spaces from a string. In standard SQL, there is the TRIM function, but in T-SQL, there are several related functions: TRIM (removing trailing and leading spaces), LTRIM (removing leading spaces), and RTRIM (removing trailing spaces).

Another very-often-used string function is SUBSTRING.

The standard SQL syntax for the SUBSTRING function looks like:

SUBSTRING(str FROM start [FOR len])

but in T-SQL, the syntax of this function looks like:

SUBSTRING(str, start, length)

There are reasons sometimes to add values coming from other columns and/or additional strings. Standard SQL enables the following syntax to do this:

As you can see, this syntax makes use of the || operator to add one string to another.

But the equivalent operator in T-SQL is the plus sign character. Look at this example:

SELECT col1 + col2  FROM tab;

In SQL Server, we also have the possibility to use the CONCAT function concatenates a list of strings:

SELECT CONCAT(col1, str1, col2, ...)  FROM tab;

We can also repeat one character several times. Standard SQL defines the function REPEAT(str, n) to do this. Transact-SQL provides the REPLICATE function. For example:


where x indicates how many times to repeat the string or character.

#9 Inequality Operator

During filtering records in a SELECT statement, sometimes we have to use an inequality operator. Standard SQL defines <> as this operator, while T-SQL allows for both the standard operator and the != operator:

SELECT col3 FROM tab WHERE col1 != col2;
#10 ISNULL Function

In T-SQL, we have the ability to replace NULL values coming from a column using the ISNULL function. This is a function that is specific to T-SQL and is not in the SQL standard.

Which Parts of DML Syntax Are Different?

In T-SQL, the basic syntax of DELETE, UPDATE, and INSERT queries is the same as the SQL standard, but differences appear in more advanced queries. Let’s look at them.

#11 OUTPUT Keyword

The OUTPUT keyword occurs in DELETE, UPDATE, and INSERT statements. It is not defined in standard SQL.

Using T-SQL, we can see extra information returned by a query. It returns both old and new values in UPDATE or the values added using INSERT or deleted using DELETE. To see this information, we have to use prefixes in INSERT, UPDATE, and DELETE.

UPDATE tab SET col='new value'
OUTPUT Deleted.col, Inserted.col;

We see the result of changing records with the previous and new values in an updated column. The SQL standard does not support this feature.

#12 Syntax for INSERT INTO ... SELECT

Another structure of an INSERT query is INSERT INTO … SELECT. T-SQL allows you to insert data from another table into a destination table. Look at this query:

INSERT INTO tab SELECT col1,col2,... FROM tab_source;

It is not a standard feature but a feature characteristic of SQL Server.

#13 FROM Clause in DELETE and UPDATE

SQL Server provides extended syntax of the UPDATE and DELETE with FROM clauses. You can use DELETE with FROM to use the rows from one table to remove corresponding rows in another table by referring to a primary key and a foreign key. Similarly, you can use UPDATE with FROM update rows from one table by referring to the rows of another table using common values (primary key in one table and foreign key in second, e.g. the same city name). Here is an example:

FROM Author
WHERE Author.Id=Book.AuthorId AND Author.Name IS NULL;

SET Book.Price=Book.Price*0.2
FROM Author
WHERE Book.AuthorId=Author.Id AND Author.Id=12;

The SQL standard doesn’t provide this syntax.


You can also use INSERT, UPDATE, and DELETE using JOIN to connect to another table. An example of this is:

DELETE ItemOrder FROM ItemOrder
JOIN Item ON ItemOrder.ItemId=Item.Id
WHERE YEAR(Item.DeliveredDate) <= 2017;

This feature is not in the SQL standard.


This article does not cover all the issues about syntax differences between the SQL standard and T-SQL using the MS SQL Server system. However, this guide helps point out some basic features characteristic only of Transact-SQL and what SQL standard syntax isn’t implemented by MS SQL Server.

Thanks for reading. If you liked this post, share it with all of your programming buddies!

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Sum Time columns in SQL

I'm writing a query to calculate when the students check-in and check-out and how much time they spend at the GYM. I have a Checking Field that contains the check-in time and a check-out field that contains the check-out, and a Duration.

I'm writing a query to calculate when the students check-in and check-out and how much time they spend at the GYM. I have a Checking Field that contains the check-in time and a check-out field that contains the check-out, and a Duration.

Check In        Check Out      Duration

12:30:00 1:30:00 1:00:00

How can I calculate check-in and check-out to obtain a duration like in the example?

T-SQL Training using Real World Scenarios:Tricks of the Trade | Simpliv

T-SQL Training using Real World Scenarios:Tricks of the Trade | Simpliv

T-SQL Training using Real World Scenarios:Tricks of the Trade

This online T-SQL video training course on Microsoft SQL Server Technology includes a lot of real world scenario which i learnt in my professional work. I have covered the course contents with the real life situation example for every topic.

Real Life situations are as follow

How to view the structure of a table
How to create your own shortcuts in SSMS
Dropping multiple tables in one go having a fixed pattern
Guidelines while performing DML Operations
How to create Solution File in SSMS and what is the need for it
How to insert a row with old identity value which was deleted
How to reset table identity
How to delete duplicate records from a table
How to select only duplicate records from a table
How to produce Date Dimension using Recursive CTE Query
How to Merge New Changes to an Existing Table
What are the advantages of Stored Procedure
How to produce running totals or cumulative totals using sql server query
How to compare current year sales with the previous year sales
How to compare current year sales since the start of the business year sales
How to switch between different SQL Servers within a query window in SSMS
How to use a single sql script file for different SQL Servers
Reducing Roundtrips using Table Valued Stored Procedure
Pagination in SQL Server using OFFSET-FETCH
Relevance of Batch in SQL Server
How to create and store customized errors
Guidelines of writing a Stored Procedure
Following are the major topics covered

Deep dive into Window Functions
Ranking Functions - (Rank, Dense_Rank Row_Number, NTile )
Aggregate Functions - (Sum,Min,Max,Avg,Count)
Analytical Functions-(Lead , Lag, First_Value, Last_Value)
Stored Procedures
Input Parameters
Output Parameters
Default Parameters
Table Valued Parameters
Common Table Expressions
Recursive Common Table Expressions
Identity Columns
Error Handling (Try….Catch)
Basic knowledge
Elementary knowledege of Transact SQL is needed like SSMS,Database Tables, Select , Join , Group By Commands ,Primary Key, Foreign Key
What will you learn
Participant's knowledge will be deepened to handle real life situations in T-SQL
Please watch the course description to see more details
To continue: