There's so many new capabilities to play around and explore in SQL Server 2019. One of the new capabilities that has caught our attention is the introduction of JSON support. In this post, I will get started in creating a JSON document using SQL Server 2019.

SQL Server 2016 will support JSON (JavaScript Object Notation). JSON is an open, text-based exchange format based on JavaScript’s object literal notation. JSON is a popular data-interchange format used in modern web and mobile applications, as well for storing unstructured data. It is supported in several DB/NoSQL engines and this improvement will enable developers to put some JSON processing logic in the SQL Server that will enable them to parse, query, analyze, and update JSON data.

Step 1

In this post, you will query a standard table and then convert it into JSON format. For this example, you are querying the standard AdventureWorks database:

SELECT TOP(2) PP.[PersonType], PP.[FirstName], PP.[MiddleName], PP.[LastName], EA.[EmailAddress]
FROM Person.Person PP
INNER JOIN [Person].[EmailAddress] EA
ON EA.[BusinessEntityID] = PP.[BusinessEntityID]

Sample output for rows inside SQL Server Management Studio looks like below:

Step 2

Now, you will add the FOR JSON AUTO support to get the same output in JSON format.

SELECT TOP(2) PP.[PersonType], PP.[FirstName], PP.[MiddleName], PP.[LastName], EA.[EmailAddress]
FROM Person.Person PP
INNER JOIN [Person].[EmailAddress] EA
ON EA.[BusinessEntityID] = PP.[BusinessEntityID]
FOR JSON AUTO

The output looks like below:

[
{
"PersonType":"EM",
"FirstName":"X",
"MiddleName":"Y",
"LastName":"Z",
"EA":[{"EmailAddress":"[email protected]"}]
},
{"PersonType":"EM",
"FirstName":"A",
"MiddleName":"B",
"LastName":"C",
"EA":[{"EmailAddress":"[email protected]"}]
}
]

Step 3

You can also generate JSON using the PATH option like:

SELECT TOP(2) PP.[PersonType], PP.[FirstName], PP.[MiddleName], PP.[LastName], EA.[EmailAddress]
FROM Person.Person PP
INNER JOIN [Person].[EmailAddress] EA
ON EA.[BusinessEntityID] = PP.[BusinessEntityID]
FOR JSON PATH

The output for this query would look little different like:

[
{"PersonType":"EM", "FirstName":"X”, “MiddleName":"Y”, “LastName":"Z”, “EmailAddress":"[email protected]"},{"PersonType":"EM”, “FirstName":"A”, “MiddleName":"B”, “LastName":"C”, “EmailAddress":"[email protected]"}
]

Now that you are able to generate few simple output using JSON keyword.

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Hi all, In this blog, I will explain to you how to apply a script or a query to multiple databases in a single execution using a database name. I get all database names, create a loop, and execute a command that I want to do.

Here I am using 2 databases, EnterpriseGL and GR8. I get all database names using a loop, and after that, I get database names one by one, put a script with a name, and execute the script.
  BEGIN TRAN 
    
  CREATE TABLE #TempGETDBLIST 
     ( 
        [NAME] NVARCHAR(255) NULL, 
        [ROWID] INT IDENTITY NOT NULL 
     ) 
           INSERT INTO #TempGETDBLIST (NAME) 
    
        --unomment which database use want to apply 
       --SELECT NAME FROM master.dbo.sysdatabases WHERE name like '%EnterpriseGL%'  -- get all GL databases 
        --SELECT NAME FROM master.dbo.sysdatabases WHERE name like '%GR8%' --get all GR8 databases 
    
          SELECT '#TempGETDBLIST',* FROM #TempGETDBLIST 
           DECLARE @Flag INT = 1 
    
  WHILE (@Flag <= (SELECT COUNT(1) FROM #TempGETDBLIST)) 
  BEGIN 
        DECLARE @ABC NVARCHAR(50), @query NVARCHAR(max) 
        SET @ABC =(SELECT Name FROM #TempGETDBLIST WHERE ROWID = @Flag) 
        SET @query = 'USE '+ @ABC +' select top 1 * from SystemConfiguration' 
    
     EXECUTE( @query ) 
     SET @Flag = @Flag + 1 
    
  END 
    
  DROP TABLE #TempGETDBLIST 
  ROLLBACK TRAN

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The SQL 'Is Null' statement returns the specified value if the expression is NULL, otherwise, it returns the expression. If the expression is NOT NULL, then this function returns the expression.

Syntax
    SELECT ISNULL(NULL, 'Csharpcorner');   

This replaces Null with the specified replacement value. 
 
A null statement doesn't specify that the Boolean result is negated. The predicate reverses its return values, returning True if the value is not Null, and False if the value is a Null statement.
 
Syntax
    ISNULL ( check_expression , replacement_value ) 

This is the expression returned if the check_expression is NULL. The replacement_value must be of a type that is implicitly convertible to the type of check_expression.
 
Using IsNull with Avg in SQL statement
 
The following example finds the average of the OrderDetails.  It substitutes the value 8 for all NULL entries in the orderId column of theOrderDetails table.
 
Syntax
    USE sample ;     
    GO     
    SELECT AVG(ISNULL(OrderId, 8))     
    FROM OrderDetails ;     
    GO     

Example

Using ISNULL in SQL
The following example selects the description, discount percentage, minimum quantity, and maximum quantity for all special offers in the sample. If the maximum quantity for a particular special offer is NULL, the MaxQty shown in the result set is 0.00. 
 
Syntax
    USE sample ;     
    GO     
    SELECT OrderId , OrderName, Orderaddress , ISNULL(OrderId, 0.00) AS 'Max Quantity'     
    FROM OrderDetails;     
    GO   

Testing for NULL in a Where clause
Do not use ISNULL to find NULL values; use IS NULL instead. The following example finds all OrderDetails that have NULL in the ordername, orderAddress, OrderDate  column. Note the space between IS and NULL.
 
Syntax
    -- Uses sample     
    SELECT OrderName, orderAddress, OrderDate     
    FROM  OrderDetails     
    WHERE OrderId  IS NULL;  

A SQL cursor is a database object that is used to retrieve data from a result set one row at a time. A SQL cursor is used when the data needs to be updated row by row. This article explains everything about SQL cursors. In this article, we will learn the following:

•     Introduction to SQL cursor
•     Cursor life cycle
•     Why and when use a cursor
•     How to implement cursors
•     What are the limitation of SQL cursor
•     How can we replace a SQL Cursor

SQL Cursor Life Cycle
The following steps are involced in a SQL cursor life cycle.

    Declaring Cursor
    A cursor is declared by defining the SQL statement.

    Opening Cursor
    A cursor is opened for storing data retrieved from the result set.

    Fetching Cursor
    When a cursor is opened, rows can be fetched from the cursor one by one or in a block to do data manipulation.

    Closing Cursor
    The cursor should be closed explicitly after data manipulation.

    Deallocating Cursor
    Cursors should be deallocated to delete cursor definition and release all the system resources associated with the cursor.

Why use a SQL Cursor?
In relational databases, operations are made on a set of rows. For example, a SELECT statement returns a set of rows which is called a result set. Sometimes the application logic needs to work with one row at a time rather than the entire result set at once. This can be done using cursors.

In programming, we use a loop like FOR or WHILE to iterate through one item at a time, the cursor follows the same approach and might be preferred because it follows the same logic.
SQL Cursor Syntax
    DECLARE cursor_name CURSOR [ LOCAL | GLOBAL ]  
    [ FORWARD_ONLY | SCROLL ] 
     [ STATIC | KEYSET | DYNAMIC | FAST_FORWARD ]  
    [ READ_ONLY | SCROLL_LOCKS | OPTIMISTIC ]  
    [ TYPE_WARNING ] FOR select_statement 
     [ FOR UPDATE [ OF column_name [ ,...n ] ] ] [;]  

Cursor Example
The following cursor is defined for retrieving employee_id and  employee_name from Employee table.The FETCH_STATUS value is 0 until there are rows.when all rows are fetched then  FETCH_STATUS becomes 1.
    use Product_Database 
    SET NOCOUNT ON;   
     
    DECLARE @emp_id int ,@emp_name varchar(20),   
        @message varchar(max);   
     
    PRINT '-------- EMPLOYEE DETAILS --------';   
     
    DECLARE emp_cursor CURSOR FOR    
    SELECT emp_id,emp_name   
    FROM Employee 
    order by emp_id;   
     
    OPEN emp_cursor   
     
    FETCH NEXT FROM emp_cursor    
    INTO @emp_id,@emp_name   
     
    print 'Employee_ID  Employee_Name'      
     
    WHILE @@FETCH_STATUS = 0   
    BEGIN   
        print '   ' + CAST(@emp_id as varchar(10)) +'           '+ 
            cast(@emp_name as varchar(20)) 
     
         
        FETCH NEXT FROM emp_cursor    
    INTO @emp_id,@emp_name   
      
    END    
    CLOSE emp_cursor;   
    DEALLOCATE emp_cursor;   

The Output of the above program will be as follows

SQL Server
What are the limitations of a SQL Cursor

A cursor is a memory resident set of pointers -- meaning it occupies memory from your system that may be available for other processes.

Cursors can be faster than a while loop but they do have more overhead.

Another factor affecting cursor speed is the number of rows and columns brought into the cursor. Time how long it takes to open your cursor and fetch statements.

Too many columns being dragged around in memory, which are never referenced in the subsequent cursor operations, can slow things down.

The cursors are slower because they update tables row by row.
How can we replace SQL Cursors
There's one replacement for cursors in SQL server joins.

Suppose we have to retrieve data from two tables simultaneously by comparing primary keys and foreign keys. In these types of problems, the cursor gives very poor performance as it processes through each and every column. On the other hand using joins in those conditions is feasible because it processes only those columns which meet the condition. So here joins are faster than cursors.

The following example explains the replacement of cursors through joins.

Suppose, we have two tables, ProductTable and Brand Table. The primary key of BrandTable is brand_id which is stored in ProductTable as foreign key brand_id. Now suppose, I have to retrieve brand_name from BrandTable using foreign key brand_id from ProductTable. In these situations cursor programs will be as follows,
    use Product_Database 
    SET NOCOUNT ON;   
     
    DECLARE @brand_id int    
    DECLARE @brand_name varchar(20)  
     
     
    PRINT '--------Brand Details --------';   
     
    DECLARE brand_cursor CURSOR FOR    
    SELECT distinct(brand_id) 
    FROM ProductTable;  
     
    OPEN brand_cursor   
     
    FETCH NEXT FROM brand_cursor    
    INTO @brand_id   
     
    WHILE @@FETCH_STATUS = 0   
    BEGIN   
        select brand_id,brand_name from BrandTable where brand_id=@brand_id 
    --(@brand_id is of ProductTable) 
         
        FETCH NEXT FROM brand_cursor    
    INTO @brand_id  
      
    END    
    CLOSE brand_cursor;   
    DEALLOCATE brand_cursor;   

The Output of the above program will be as follows

SQL Server
The same program can be done using joins as follows,
Select distinct b.brand_id,b.brand_name from BrandTable b inner join
ProductTable p on b.brand_id=p.brand_id

The Output of the above program will be as follows

SQL Server
As we can see from the above example, using joins reduces the lines of code and gives faster performance in case huge records need to be processed.

The Unique constraint statement ensures that all values in a column are different. Both the Unique and Primary Key constraints provide a guarantee for uniqueness for a column or set of columns. A Primary key constraint automatically has a unique constraint in SQL. However, you can have many unique constraints per table, but only one primary key constraint per table.

 
We can create a unique constraint in SQL Server 2019 (15.x) by using SQL server management studio or SQL to ensure no duplicate values are entered in specific columns that do not participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index in SQL.  
 
SQL Server Unique constraints allow you to ensure that the data stored in a column, or a group of columns, is unique among the rows in a table. 
 
Syntax
    CREATE TABLE EmployeeName  (       
     EmpID int NOT NULL UNIQUE,       
      EmpName varchar(255) NOT NULL,        
    );    

The above query creates a table with the name "EmployeeName"  and column name EmpID which is both Not Null and Unique(i.e we cannot have empty or duplicate data) and EmpName
 
Using Unique constraint on create table statement
 
Syntax
    CREATE TABLE Employee  (     
     EmpID int NOT NULL UNIQUE,     
      EmpName varchar(255) NOT NULL,     
      EmpFirstName varchar(255),     
      EmpLastname varchar(255),     
      EmpAge int     
    );    

The above query created a table with the name "Employee" and the first column name EmpId is Not NULL and UNIQUE, other column name EmpName,EmpFirstName,EmpLastname, EmpAge
 
Using Unique constraint on alter table
 
Syntax
    ALTER TABLE Employee      
    ADD UNIQUE (EmpID);    

 The above query with add a column EmpID and make it UNIQUE.
 
"Unique" is used to signify a Unique constraint, and also to define a unique name a Unique constraint,on multiple columns.
 
Syntax
    ALTER TABLE Employee      
    ADD CONSTRAINT UC_Employee UNIQUE (EmpID,EmpLastName);    

The above query will add EmpID and EmpLastName columns into the UC_Employee table, with the Unique Constraint
 
Using Drop a Unique constraint statement
 
Use the following example to drop a Unique constraint:
 
Syntax 
    ALTER TABLE Employee      
    DROP CONSTRAINT UC_Employee;    

The above query will remove the "Unique" Constrain from the Employee table.
 
Using SQL Server Management Studio in Unique Constraint
 
To create a unique constraint statement:
    In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.
    On the Table Designer menu, click Indexes/Keys.
    In the Indexes/Keys dialog box, click Add.
    In the grid under General, click Type and choose Unique Key from the drop-down list box to the right of the property.
    On the File menu, click Save table name.

Using unique constraint in SQL
 
To create a unique constraint,

    In Object Explorer, connect to an instance of Database Engine.
    On the Standard bar, click New Query.
    Copy and paste the following example into the query window and click Execute. This example creates the table SampleDetails and creates a unique constraint on the column TransactionID.

Syntax
    USE sample ;       
    GO       
    CREATE TABLE SampleDetails       
     (       
       TransactionID int NOT NULL,        
       CONSTRAINT AK_TransactionID UNIQUE(TransactionID)        
    );        
    GO      

The above query will create a table SampleDetails in the sample database, with TransactionID and the AK_TransactionID constraint which makes TransactionID unique
 
To create a unique constraint on an existing table
    In Object Explorer, connect to an instance of Database Engine.
    On the Standard bar, click New Query.
    Copy and paste the following example into the query window and click Execute. The example creates a unique constraint on the columns PasswordHash and PasswordSalt in the table Person.Password. 

Syntax
    USE sample         
    GO       
    ALTER TABLE Person.Password        
    ADD CONSTRAINT AK_Password UNIQUE (PasswordHash, PasswordSalt);        
    GO       

The above query appends the table Person.Password in the sample database , with the AK_Password CONSTRAINT which makes PasswordHash amd PasswordSalt unique.
 
To create a unique constraint in a new table
    In Object Explorer, connect to an instance of Database Engine.
    On the Standard bar, click New Query.
    Copy and paste the following example into the query window and click Execute. The example creates a table and defines a unique constraint on the column TransactionID.

Syntax 
    USE sample;       
    GO       
    CREATE TABLE Production.TransactionHistoryArchive2       
    (       
       TransactionID int NOT NULL,       
       CONSTRAINT AK_TransactionID UNIQUE(TransactionID)       
    );       
    GO      

The above query uses the sample database and creates a table with the name "Production.TransactionHistoryArchive2" and column name TransactionID and CONSTRAINT AK_TransactionID UNIQUE column name is TransactionID.

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Sometimes, we have a scenario when we want to get a common set of rows from 2 different result sets. For example, we have 2 queries and both returns employees record. If we want to find who all employees are present in both result sets, that time we can use INTERSECT to get the result. Below is the graphical representation of how INTERSECT works.

If you see in the above graphical representation, the left 2 circles have the B and C letters in common. The left side picture shows how the 2 circles have B and C letters common which is nothing but an intersection.

Now let's see how it works in the database.

So we are going to create 2 tables, EmpTable and ManagerEmp and then we will insert records in them.
CREATE TABLE EmpTable(EmpName   VARCHAR(50),City    VARCHAR(50),Title   VARCHAR(50)) 
CREATE TABLE ManagerTable(EmpName   VARCHAR(50),City    VARCHAR(50),Title   VARCHAR(50)) 
 
INSERT INTO EmpTable 
SELECT EmpName='John',City='Stamford',Title='Operator' 
 
INSERT INTO EmpTable 
SELECT EmpName='Luis',City='Danbury',Title='Electrical Engineer' 
 
INSERT INTO EmpTable 
SELECT EmpName='Smith',City='Wilton',Title='Driver' 
 
 
INSERT INTO ManagerTable 
SELECT EmpName='Mike',City='Wilton',Title='Driver' 
 
INSERT INTO ManagerTable 
SELECT EmpName='Smith',City='Wilton',Title='Driver' 
 
INSERT INTO ManagerTable 
SELECT EmpName='Jonathan',City='Armonk',Title='Accountant' 
 
INSERT INTO ManagerTable 
SELECT EmpName='Warner',City='Stamford',Title='Customer Service' 
 
INSERT INTO ManagerTable 
SELECT EmpName='Luis',City='Danbury',Title='Electrical Engineer' 

Now run below the query to find out the common employees in both tables.
SELECT * FROM EmpTable 
 
SELECT * FROM ManagerTable 
 
-- INTERSECTION 
SELECT * FROM EmpTable 
INTERSECT 
SELECT * FROM ManagerTable 

Here is the output.

If you see below, in both the "EmpTable" and the "ManagerTable" tables, Luis and Smith both are employees. To join these 2 queries with INTERSECT, it gave these 2 names.

One thing is to remember here is both the tables/result sets should have the same columns and the same datatype for those columns, otherwise, it may give you "Conversion failed when converting...." if the data type does not match.

ERROR - "Cannot resolve the collation conflict between SQL_Latin1_General_CP1_CI_AS and Latin1_General_CI_AS_KS_WS within the up to operation."

Don’t panic if you get this error while joining your tables. there's an easy way to solve this. It happens because of the different collation settings on 2 columns we are joining.

The first step is to figure out what are the two collations that have caused the conflicts.
Let us assume that collation has not been set at the column level and instead at the db level. Then, we've to execute two straightforward statements as below.
Statements

• Select DATABASEPROPERTYYEX('DB1',N'Collation')
  
• Select DATABASEPROPERTYYEX('DB2',N'Collation') 

One more thing to make a note of here is that if you are on SharePoint, you will get an error as following.

Latin_General_CI_AS_KS_WS.
 
If you are on any other database and use the default settings, you may get this SQL_Latin_General_CP1_CI_AS.

Now, we have to do something similar to CAST, called Collate (FOR Collation).

Refer to the example below.
      select * from Demo1.dbo.Employee emp 
    join Demo2.dbo.Details dt 
    on (emp.email =dt.email COLLATE SQL_Latin_General_CP1_CI_AS) 

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We know that sorting can be one of the most expensive things in an execution plan as shown below. However, we continue to do ORDER BYs repeatedly. Yes, I 100% agree that there is a need to sort a results set and that this should be done in the procedure for good reason, but my concern is having multiple sorts, erroneous sorts, and the sorts that can be done elsewhere. These are the ones that waste resources and can stifle performance.

Many of us writing procedures tend to write in code blocks. We write the SELECT, JOINS, FROMs and WHERES then immediately follow it up with and ORDER BY as a way to check result sets before moving onto the next block of code. I admit I do this almost every time. But what most developers do not do is remove unneeded ORDER BYs that are not required. This is very costly and can lead to suboptimal performance not only of your procedure but also for TEMPDB as this is where all sorting takes place.
Do you sort in your procedures that are used for data consumers like reports, ETL or an application? If you do, I ask, why are you sorting in the procedure and not in the consumer ? Many report end users will resort the data in Excel, or the report itself gives parameters for custom sorts or the data doesn’t need a sort at all. Why are you wasting resources on the SQL Server side just for it to be nullified? By removing unneeded sorts or performing the sort in the application tier you can have big performance gains. I would rather have a report, ETL process or application take the performance hit then a procedure.
 
Let’s look at one of the procedures that are available in AdventureWorks2016CPT3 called uspGetOrderTrackingBySalesOrderID. We will run it using the example execution in the code and then remove the ORDER BY, compile and rerun. We will be able to see clearly see the difference.
    USE [AdventureWorks2016CTP3] 
     
    GO 
     
    /****** Object:  StoredProcedure [dbo].[uspGetOrderTrackingBySalesOrderID]    Script Date: 1/28/2020 11:31:16 AM ******/ 
     
    SET ANSI_NULLS ON 
     
    GO 
    SET QUOTED_IDENTIFIER ON 
    GO 
     
    ALTER PROCEDURE [dbo].[uspGetOrderTrackingBySalesOrderID] 
     
       @SalesOrderID [int] NULL 
    AS 
    BEGIN 
    /* Example: 
     
          exec dbo.uspGetOrderTrackingBySalesOrderID 53498 
    */ 
       SET NOCOUNT ON; 
       SET STATISTICS IO, TIME ON 
     
       SELECT 
          ot.SalesOrderID, 
          ot.CarrierTrackingNumber, 
          ot.OrderTrackingID, 
          ot.TrackingEventID, 
          te.EventName, 
          ot.EventDetails, 
          ot.EventDateTime 
       FROM 
          Sales.OrderTracking ot, 
          Sales.TrackingEvent te 
       WHERE 
          ot.SalesOrderID = @SalesOrderID AND 
          ot.TrackingEventID = te.TrackingEventID 
       --ORDER BY 
       --   ot.SalesOrderID, 
       --   ot.TrackingEventID; 
    END; 

Plan with ORDER BY

Plan without ORDER BY and Query Store graph showing the difference in duration between the two. You can clearly see the performance improvement, and this was just one sort in a very simple procedure. Take a moment and consider the sorting that happens in your code. I’d ask that when writing store procedures, doing code reviews or performance tuning that you take a second to ask why the sorts are being done in the data tier and if they can be performed elsewhere. You can see get some performance gains not only in your code but in TEMPDB as well when sorting is reigned in.

Nonclustered HASH Index
This index is used to access the In-Memory version of the table, called a Hash. These are great for predicates that are singleton lookups and not ranges of values. These are optimized for seeks of equality values. For example, WHERE Name = ‘Joe’. Something to keep in mind when determining what to include in your indexes is this; if your query has two or more fields as your predicate and your index only consists of one of those fields, you will get a scan. It will not seek on that one field that was included.

Nonclustered Index
These are also used to access the In-Memory version of the table however, these are optimized for range values such as less than and equal to, inequality predicates and sorts orders. Examples are WHERE DATE between ‘20190101’ and ‘20191231’ and WHERE DATE <> ‘20191231’. These indexes do not require a bucket count or fixed memory amount. The memory consumed by these indexes are determined by the actual row counts and size of the indexed key columns which makes it a simpler to create.

SQL Comments statement can make your application easier for you to read and maintain. For example, we can include a comment in a statement that describes the purpose of the statement within your application with the exception of hints, comments within SQL. The statement does not affect the statement execution. Please refer to using hints on using this particular form of comment statement. 
 
A comment can appear between any keywords, parameters, or punctuation marks in a statement. You can include a comment in a statement in two ways:

• Begin the comment with a slash and an asterisk (/*). Proceed with the text of the comment. This text can span multiple lines.
• End the comment with an asterisk and a slash (*/). The opening and terminating characters need not be separated from the text by a space or a line break.
• Begin the comment with -- (two hyphens). Proceed with the text of the comment. This text cannot extend to a new line. End the comment with a line break.

Some of the tools used to enter SQL have additional restrictions. For example, if you are using SQL*plus, by default you cannot have a blank line inside a multiline comment.
 
For more information, please refer to the documentation for the tool you use as an interface to the database. A SQL statement can contain multiple comments of both styles. The text of a comment can contain any printable characters in your database character set.
 
The comment statement indicates the user-provided text. Comments can be inserted on a separate line, nested at the end of a SQL command line, or within a SQL statement. The server does not evaluate the comment. 
SQL Comment uses the two hyphens (--) for single-line or nested comments. Comments inserted with -- are terminated by a new line, which is specified with a carriage return. Character (U+000A), line feed character (U+000D), or a combination of the two in SQL comments.
 
There is no maximum length for comments. The following table lists the keyboard shortcuts that you can use to comment or uncomment text.
 
Syntax
    -- text_of_comment    

Examples
The following example uses the -- commenting characters.
 
Syntax
    -- Choose the sample database.     
    USE sample;     
    GO     
    -- Choose all columns and all rows from the Address table.     
    SELECT *     
    FROM OrderDetails     
    ORDER BY OrderId  ASC; -- We do not have to specify ASC because      
    -- that is the default.     

SQL Single Line Comments
Single line comments start with --. Any text between -- and the end of the line will be ignored (will not be executed). The following example uses a single-line comment as an explanation.
 
Syntax 
   --Select all:   
    SELECT * FROM OrderDetails ; 

The following example uses a single-line comment to ignore the end of a line.
 
Syntax
    SELECT * FROM OrderDetails -- WHERE OrderName='Coffee';  

The following example uses a single-line comment to ignore a statement.
 
Syntax
    --SELECT * FROM OrderDetails;   
    SELECT * FROM OrderDetails ;  

SQL Multi-line Comments
SQL Multi-line comments start with /* and end with */. Any text between /* and */ will be ignored. The following example uses a multi-line comment as an explanation.
 
Syntax 
    /*Select all the columns   
    of all the records   
    in the OrderDetails table:*/   
    SELECT * FROM OrderDetails;  

 The following example uses a multi-line comment to ignore many statements.
 
Syntax
    /*SELECT * FROM Customers;   
    SELECT * FROM Products;   
    SELECT * FROM Orders;   
    SELECT * FROM Categories;*/   
    SELECT * FROM OrderDetails;  

To ignore just a part of a statement, also use the /* */ comment. The following example uses a comment to ignore part of a line.
 
Syntax
    SELECT CustomerName, /*City,*/ Country FROM Customers;  

The following example uses a comment to ignore part of a statement
 
Syntax
    SELECT * FROM OrderDetails WHERE (OrderName LIKE 'L%'   
    OR OrderName LIKE 'R%' /*OR OrderName  LIKE 'S%'   
    OR OrderName LIKE 'T%'*/ OR OrderName LIKE 'W%')   
    AND OrderName ='Mango'   
    ORDER BY OrderrAddress;  

Summary
In this article, you learned how to use a SQL Comments statement with various options.