Difference Between a Statement and a Query in SQL

Difference between a statement and a query in SQL

A statement is any text that the database engine recognizes as a valid command. As of SQL-92:

An SQL-statement is a string of characters that conforms to the format and syntax rules specified in this international standard.

A query is a statement that returns a recordset (possibly empty).

How can I call a chunk of SQL code made by more than one statement where statements are separated by a semicolon (;)? Who already replied can edit his answer. Many thanks!

A series of SQL statements sent to the server at once is called a batch.

Not all SQL engines required the statements in a batch to be semicolon delimited. SQL Server, for instance, generally does not and breaks the statements based on context. CTE statements starting with WITH are a notable exception.

What is difference between clause, command, statement and query in SQL?

I will give you the terminology used with SQL Server. Some of these (certainly the ones in your question) will be extremely common across all database systems, some may be system specific.

From highest level to lowest:

• Script. A single file containing SQL code. May contain multiple batches

• Batch. A batch is the unit in which work is submitted to the server. In SQL Server, each batch is (normally) delimited by GO. Splitting the script into batches is a job performed by client tools. A Batch may contain multiple statements.

• Statement (a.k.a Command¹ or Query). This is the smallest unit of individual work that the server will work with. I.e. Every statement is something "complete", which will cause the server to perform some work and may result in data being modified and/or a result set being returned. Typically, the server will compile each statement individually (but may do each compilation for every statement in a Batch before any of them are executed).

• Clause². A Clause is a subunit of a statement - but, beware, some statements may consist of only a single Clause, which may appear to muddy the waters a little. For example, some database systems will accept SELECT 10; as a Query. This is a SELECT statement consisting only of a SELECT clause. Multiple statement types may use the same clause types. E.g. Both SELECT and DELETE statements may contain a WHERE clause. Also, most statements will have a clause that shares the same name.

• Expression². An expression is something that produces a scalar value (Note though that, in most contexts, this is understood to be "one scalar value per row", not "one scalar value in total")

  • Predicate. A boolean expression, most often encountered in WHERE clauses, WHEN clauses and CHECK constraints. These are especially called out because not all database systems support a user visible boolean data type, and so they're not always treated the same as other expressions.

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¹Many client libraries will expose some kind of command object for submitting queries to the database system. However, to muddy the waters further, many of these will accept a batch. Nevertheless, command seems to have stuck as having a similar meaning as statement, possible because in the vast majority of cases, the command object isn't used for multiple statements in one go.

²Note that to some extent, these share the same level. A SELECT clause may contain a CASE expression, that consists of multiple WHEN and THEN clauses.

Difference between sql statements and clause

The following statement:

SELECT foo FROM bar JOIN quux WHERE x = y;

is made up of the following clauses:

• WHERE x = y
• SELECT foo
• FROM bar
• JOIN quux

What is the difference between a query and transaction in SQL?

A query usually refers to a SELECT statement, but may also refer to data modifications as well -- UPDATE, INSERT, DELETE, and MERGE are common DML ("data modification language") statements. Personally, I would call these four operations DML statements and reserve query for SELECT statements; I find this a useful distinction.

Databases implement a set of properties called ACID properties. These basically say that any SQL statement sees consistent data, regardless of what else is going on in the database. A simple way to think of about these is that all operations are serialized -- one statement completes before another begins, even in a multi-user environment. Serialization guarantees that operations are isolated from each other. In practice, serialization is very expensive and databases have other mechanisms to ensure integrity, but it is a useful abstraction when learning about the concepts.

What are transactions? Transactions are the mechanism that databases use to ensure the integrity of the data when data is being modified. Transactions often consist of one statement that modifies the data. But that is not necessary. In fact, you can have a complex set of data transformations within a single transaction.

The three key operations on a transactions are:

• BEGIN TRANSACTION: Tell the database that a transaction is beginning. All changes within the transaction are invisible to other users while the transaction is "active".
• COMMIT TRANSACTION: Make all the changes visible in the database. Conceptually, this happens instantaneously. The database guarantees that other users will not see partial changes to the data (i.e. enforces data integrity).
• ROLLBACK TRANSACTION: Undo all the work. No other users ever see the changes.

Note: Many databases have options to weaken the data integrity. These can be useful for performance reasons for users who know what they are doing.

I should note that if all operations on a database are SELECT statements, then transactions are not necessary. The data is not changing, so the view of the data is consistent. So transactions are generally associated with DML statements.

Need help understanding the difference between a script format and query

In this context it sounds like they want to see any set of commands that will be changing (INSERT, UPDATE, DELETE, etc.) data and are not concerned with any queries you're running to simply return or review data.

I suspect more people use these terms interchangeably than you think.

It's a very good idea to go back to this department unashamed and ask for clarification as different teams use different terms. I think you will find they're asking you to plan any data-changing action out in advance, put your commands in sequence in a file (like a .sql file if you work in Management studio, etc.) and forward that file to them for review.

Asking around was a great move. You cannot be too careful with these things!

Difference between 2 selects in SQL

Try this:

SELECT * 
FROM table
WHERE value NOT IN ( SELECT value FROM table WHERE user = 1)

Difference between if and when statements in SQL

IF statement controls the flow and decides what has to be evaluated next, based on the condition, while CASE is a function which returns the desired value.

The CASE statement is more readable than IF statement when you compare a single expression against a range of unique values and more efficient as well.

What's the difference of SELECT and SELECT IN in sql?

I ran execution plan on 3 different queries.

First query: Using UNION

Second query: Using UNION ALL

Third query: Using IN

USE AdventureWorksLT2012

-- First query using UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 716
UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 727
UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 770

-- Second query using UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 716
UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 727
UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 770

-- Third query using IN
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID IN(716, 727, 770)

As you can see the UNION is using 53% (Because UNION tries to delete duplicates), UNION ALL is costing 34% and IN costs 14% of whole batch

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