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SQL Optimization Tips and Index Optimization tips

QUERIES

  1. 2 tables

Employee

Phone

empid
empname
salary
mgrid

empid
phnumber

  1. Select all employees who doesn't have phone?

SELECT empname
FROM Employee
WHERE (empid NOT IN
(SELECT DISTINCT empid
FROM phone))

  1. Select the employee names who is having more than one phone numbers.
  2. Select empname where select empid where
    SELECT empname
    FROM employee
    WHERE (empid IN
    (SELECT empid
    FROM phone
    GROUP BY empid
    HAVING COUNT(empid) > 1))
  3. Select the details of 3 max salaried employees from employee table.
    SELECT TOP 3 empid, salary
    FROM employee
    ORDER BY salary DESC
  4. Display all managers from the table. (manager id is same as emp id)
    SELECT empname
    FROM employee
    WHERE (empid IN
    (SELECT DISTINCT mgrid
    FROM employee))
  5. Write a Select statement to list the Employee Name, Manager Name under a particular manager?
    SELECT e1.empname AS EmpName, e2.empname AS ManagerName
    FROM Employee e1 INNER JOIN
    Employee e2 ON e1.mgrid = e2.empid
    ORDER BY e2.mgrid
  6. 2 tables emp and phone.
    emp fields are – empid, name
    Ph fields are – empid, ph (office, mobile, home). Select all employees who doesn't have any ph nos.
    SELECT *
    FROM employee LEFT OUTER JOIN
    phone ON employee.empid = phone.empid
    WHERE (phone.office IS NULL OR phone.office = ' ')
    AND (phone.mobile IS NULL OR phone.mobile = ' ')
    AND (phone.home IS NULL OR phone.home = ' ')
  7. Find employee who is living in more than one city.
    Two Tables:

Emp

City

Empid
empName
Salary

Empid
City

  1. SELECT empname, fname, lname
    FROM employee
    WHERE (empid IN
    (SELECT empid
    FROM city
    GROUP BY empid
    HAVING COUNT(empid) > 1))
  2. Find all employees who is living in the same city. (table is same as above)
    SELECT fname
    FROM employee
    WHERE (empid IN
    (SELECT empid
    FROM city a
    WHERE city IN
    (SELECT city
    FROM city b
    GROUP BY city
    HAVING COUNT(city) > 1)))
  3. There is a table named MovieTable with three columns – moviename, person and role. Write a query which gets the movie details where Mr. Amitabh and Mr. Vinod acted and their role is actor.
    SELECT DISTINCT m1.moviename
    FROM MovieTable m1 INNER JOIN
    MovieTable m2 ON m1.moviename = m2.moviename
    WHERE (m1.person = 'amitabh' AND m2.person = 'vinod' OR
    m2.person = 'amitabh' AND m1.person = 'vinod') AND (m1.role = 'actor') AND (m2.role = 'actor')
    ORDER BY m1.moviename
  4. There are two employee tables named emp1 and emp2. Both contains same structure (salary details). But Emp2 salary details are incorrect and emp1 salary details are correct. So, write a query which corrects salary details of the table emp2
    update a set a.sal=b.sal from emp1 a, emp2 b where a.empid=b.empid
  5. Given a Table named “Students” which contains studentid, subjectid and marks. Where there are 10 subjects and 50 students. Write a Query to find out the Maximum marks obtained in each subject.
  6. In this same tables now write a SQL Query to get the studentid also to combine with previous results.
  7. Three tables – student , course, marks – how do go at finding name of the students who got max marks in the diff courses.
    SELECT student.name, course.name AS coursename, marks.sid, marks.mark
    FROM marks INNER JOIN
    student ON marks.sid = student.sid INNER JOIN
    course ON marks.cid = course.cid
    WHERE (marks.mark =
    (SELECT MAX(Mark)
    FROM Marks MaxMark
    WHERE MaxMark.cID = Marks.cID))
  8. There is a table day_temp which has three columns dayid, day and temperature. How do I write a query to get the difference of temperature among each other for seven days of a week?
    SELECT a.dayid, a.dday, a.tempe, a.tempe – b.tempe AS Difference
    FROM day_temp a INNER JOIN
    day_temp b ON a.dayid = b.dayid + 1
    OR
    Select a.day, a.degree-b.degree from temperature a, temperature b where a.id=b.id+1
  9. There is a table which contains the names like this. a1, a2, a3, a3, a4, a1, a1, a2 and their salaries. Write a query to get grand total salary, and total salaries of individual employees in one query.
    SELECT empid, SUM(salary) AS salary
    FROM employee
    GROUP BY empid WITH ROLLUP
    ORDER BY empid
  10. How to know how many tables contains empno as a column in a database?
    SELECT COUNT(*) AS Counter
    FROM syscolumns
    WHERE (name = 'empno')
  11. Find duplicate rows in a table? OR I have a table with one column which has many records which are not distinct. I need to find the distinct values from that column and number of times it’s repeated.
    SELECT sid, mark, COUNT(*) AS Counter
    FROM marks
    GROUP BY sid, mark
    HAVING (COUNT(*) > 1)
  12. How to delete the rows which are duplicate (don’t delete both duplicate records).
    SET ROWCOUNT 1
    DELETE yourtable
    FROM yourtable a
    WHERE (SELECT COUNT(*) FROM yourtable b WHERE b.name1 = a.name1 AND b.age1 = a.age1) > 1
    WHILE @@rowcount > 0
    DELETE yourtable
    FROM yourtable a
    WHERE (SELECT COUNT(*) FROM yourtable b WHERE b.name1 = a.name1 AND b.age1 = a.age1) > 1
    SET ROWCOUNT 0
  13. How to find 6th highest salary
    SELECT TOP 1 salary
    FROM (SELECT DISTINCT TOP 6 salary
    FROM employee
    ORDER BY salary DESC) a
    ORDER BY salary
  14. Find top salary among two tables
    SELECT TOP 1 sal
    FROM (SELECT MAX(sal) AS sal
    FROM sal1
    UNION
    SELECT MAX(sal) AS sal
    FROM sal2) a
    ORDER BY sal DESC
  15. Write a query to convert all the letters in a word to upper case
    SELECT UPPER('test')
  16. Write a query to round up the values of a number. For example even if the user enters 7.1 it should be rounded up to 8.
    SELECT CEILING (7.1)
  17. Write a SQL Query to find first day of month?
    SELECT DATENAME(dw, DATEADD(dd, – DATEPART(dd, GETDATE()) + 1, GETDATE())) AS FirstDay

Datepart

Abbreviations

year

yy, yyyy

quarter

qq, q

month

mm, m

dayofyear

dy, y

day

dd, d

week

wk, ww

weekday

dw

hour

hh

minute

mi, n

second

ss, s

millisecond

ms

  1. Table A contains column1 which is primary key and has 2 values (1, 2) and Table B contains column1 which is primary key and has 2 values (2, 3). Write a query which returns the values that are not common for the tables and the query should return one column with 2 records.
    SELECT tbla.a
    FROM tbla, tblb
    WHERE tbla.a <>
    (SELECT tblb.a
    FROM tbla, tblb
    WHERE tbla.a = tblb.a)
    UNION
    SELECT tblb.a
    FROM tbla, tblb
    WHERE tblb.a <>
    (SELECT tbla.a
    FROM tbla, tblb
    WHERE tbla.a = tblb.a)

    OR (better approach)

    SELECT a
    FROM tbla
    WHERE a NOT IN
    (SELECT a
    FROM tblb)
    UNION ALL
    SELECT a
    FROM tblb
    WHERE a NOT IN
    (SELECT a
    FROM tbla)

  2. There are 3 tables Titles, Authors and Title-Authors (check PUBS db). Write the query to get the author name and the number of books written by that author, the result should start from the author who has written the maximum number of books and end with the author who has written the minimum number of books.
    SELECT authors.au_lname, COUNT(*) AS BooksCount
    FROM authors INNER JOIN
    titleauthor ON authors.au_id = titleauthor.au_id INNER JOIN
    titles ON titles.title_id = titleauthor.title_id
    GROUP BY authors.au_lname
    ORDER BY BooksCount DESC
  3. UPDATE emp_master
    SET emp_sal =
    CASE
    WHEN emp_sal > 0 AND emp_sal <= 20000 THEN (emp_sal * 1.01)
    WHEN emp_sal > 20000 THEN (emp_sal * 1.02)
    END
  4. List all products with total quantity ordered, if quantity ordered is null show it as 0.
    SELECT name, CASE WHEN SUM(qty) IS NULL THEN 0 WHEN SUM(qty) > 0 THEN SUM(qty) END AS tot
    FROM [order] RIGHT OUTER JOIN
    product ON [order].prodid = product.prodid
    GROUP BY name
    Result:
    coke 60
    mirinda 0
    pepsi 10
  5. ANY, SOME, or ALL?
    ALL means greater than every value–in other words, greater than the maximum value. For example, >ALL (1, 2, 3) means greater than 3.
    ANY means greater than at least one value, that is, greater than the minimum. So >ANY (1, 2, 3) means greater than 1. SOME is an SQL-92 standard equivalent for ANY.
  6. IN & = (difference in correlated sub query)

INDEXES

 

  1. What is Index? It’s purpose?
    Indexes in databases are similar to indexes in books. In a database, an index allows the database program to find data in a table without scanning the entire table. An index in a database is a list of values in a table with the storage locations of rows in the table that contain each value. Indexes can be created on either a single column or a combination of columns in a table and are implemented in the form of B-trees. An index contains an entry with one or more columns (the search key) from each row in a table. A B-tree is sorted on the search key, and can be searched efficiently on any leading subset of the search key. For example, an index on columns A, B, C can be searched efficiently on A, on A, B, and A, B, C.
  2. Explain about Clustered and non clustered index? How to choose between a Clustered Index and a Non-Clustered Index?
    There are clustered and nonclustered indexes. A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
    A nonclustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf nodes of a nonclustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
    Consider using a clustered index for:
    • Columns that contain a large number of distinct values.
    • Queries that return a range of values using operators such as BETWEEN, >, >=, <, and <=.
    • Columns that are accessed sequentially.
    • Queries that return large result sets.
      Non-clustered indexes have the same B-tree structure as clustered indexes, with two significant differences:
    • The data rows are not sorted and stored in order based on their non-clustered keys.
    • The leaf layer of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. Each index row contains the non-clustered key value and one or more row locators that point to the data row (or rows if the index is not unique) having the key value.
    • Per table only 249 non clustered indexes.
  3. Disadvantage of index?
    Every index increases the time in takes to perform INSERTS, UPDATES and DELETES, so the number of indexes should not be very much.
  4. Given a scenario that I have a 10 Clustered Index in a Table to all their 10 Columns. What are the advantages and disadvantages?
    A: Only 1 clustered index is possible.
  5. How can I enforce to use particular index?
    You can use index hint (index=<index_name>) after the table name.
    SELECT au_lname FROM authors (index=aunmind)
  6. What is Index Tuning?
    One of the hardest tasks facing database administrators is the selection of appropriate columns for non-clustered indexes. You should consider creating non-clustered indexes on any columns that are frequently referenced in the WHERE clauses of SQL statements. Other good candidates are columns referenced by JOIN and GROUP BY operations.
    You may wish to also consider creating non-clustered indexes that cover all of the columns used by certain frequently issued queries. These queries are referred to as “covered queries” and experience excellent performance gains.
    Index Tuning is the process of finding appropriate column for non-clustered indexes.
    SQL Server provides a wonderful facility known as the Index Tuning Wizard which greatly enhances the index selection process.
  7. Difference between Index defrag and Index rebuild?
    When you create an index in the database, the index information used by queries is stored in index pages. The sequential index pages are chained together by pointers from one page to the next. When changes are made to the data that affect the index, the information in the index can become scattered in the database. Rebuilding an index reorganizes the storage of the index data (and table data in the case of a clustered index) to remove fragmentation. This can improve disk performance by reducing the number of page reads required to obtain the requested data
    DBCC INDEXDEFRAG – Defragments clustered and secondary indexes of the specified table or view.
    **
  8. What is sorting and what is the difference between sorting & clustered indexes?
    The ORDER BY clause sorts query results by one or more columns up to 8,060 bytes. This will happen by the time when we retrieve data from database. Clustered indexes physically sorting data, while inserting/updating the table.
  9. What are statistics, under what circumstances they go out of date, how do you update them?
    Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with non-unique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query.
    Some situations under which you should update statistics:
    1) If there is significant change in the key values in the index
    2) If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated
    3) Database is upgraded from a previous version
  10. What is fillfactor? What is the use of it ? What happens when we ignore it? When you should use low fill factor?
    When you create a clustered index, the data in the table is stored in the data pages of the database according to the order of the values in the indexed columns. When new rows of data are inserted into the table or the values in the indexed columns are changed, Microsoft® SQL Server™ 2000 may have to reorganize the storage of the data in the table to make room for the new row and maintain the ordered storage of the data. This also applies to nonclustered indexes. When data is added or changed, SQL Server may have to reorganize the storage of the data in the nonclustered index pages. When a new row is added to a full index page, SQL Server moves approximately half the rows to a new page to make room for the new row. This reorganization is known as a page split. Page splitting can impair performance and fragment the storage of the data in a table.
    When creating an index, you can specify a fill factor to leave extra gaps and reserve a percentage of free space on each leaf level page of the index to accommodate future expansion in the storage of the table's data and reduce the potential for page splits. The fill factor value is a percentage from 0 to 100 that specifies how much to fill the data pages after the index is created. A value of 100 means the pages will be full and will take the least amount of storage space. This setting should be used only when there will be no changes to the data, for example, on a read-only table. A lower value leaves more empty space on the data pages, which reduces the need to split data pages as indexes grow but requires more storage space. This setting is more appropriate when there will be changes to the data in the table.

    DATA TYPES

  11. What are the data types in SQL

bigint

Binary

bit

char

cursor

datetime

Decimal

float

image

int

money

Nchar

ntext

nvarchar

real

smalldatetime

Smallint

smallmoney

text

timestamp

tinyint

Varbinary

Varchar

uniqueidentifier

 

  1. Difference between char and nvarchar / char and varchar data-type?
    char[(n)] – Fixed-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is n bytes. The SQL-92 synonym for char is character.
    nvarchar(n) – Variable-length Unicode character data of n characters. n must be a value from 1 through 4,000. Storage size, in bytes, is two times the number of characters entered. The data entered can be 0 characters in length. The SQL-92 synonyms for nvarchar are national char varying and national character varying.
    varchar[(n)] – Variable-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is the actual length in bytes of the data entered, not n bytes. The data entered can be 0 characters in length. The SQL-92 synonyms for varchar are char varying or character varying.

 

  1. GUID datasize?
    128bit
  2. How GUID becoming unique across machines?
    To ensure uniqueness across machines, the ID of the network card is used (among others) to compute the number.
  3. What is the difference between text and image data type?
    Text and image. Use text for character data if you need to store more than 255 characters in SQL Server 6.5, or more than 8000 in SQL Server 7.0. Use image for binary large objects (BLOBs) such as digital images. With text and image data types, the data is not stored in the row, so the limit of the page size does not apply.All that is stored in the row is a pointer to the database pages that contain the data.Individual text, ntext, and image values can be a maximum of 2-GB, which is too long to store in a single data row.

    JOINS

  4. What are joins?
    Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
  5. How many types of Joins?
    Joins can be categorized as:
  • Inner joins (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.
    Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.
  • Outer joins. Outer joins can be a left, a right, or full outer join.
    Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:
    • LEFT JOIN or LEFT OUTER JOIN -The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.
    • RIGHT JOIN or RIGHT OUTER JOIN – A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.
    • FULL JOIN or FULL OUTER JOIN – A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.
  • Cross joins – Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products. (A Cartesian join will get you a Cartesian product. A Cartesian join is when you join every row of one table to every row of another table. You can also get one by joining every row of a table to every row of itself.)
  1. What is self join?
    A table can be joined to itself in a self-join.
  2. What are the differences between UNION and JOINS?
    A join selects columns from 2 or more tables. A union selects rows.
  3. Can I improve performance by using the ANSI-style joins instead of the old-style joins?
    Code Example 1:
    select o.name, i.name
    from sysobjects o, sysindexes i
    where o.id = i.id
    Code Example 2:
    select o.name, i.name
    from sysobjects o inner join sysindexes i
    on o.id = i.id
    You will not get any performance gain by switching to the ANSI-style JOIN syntax.
    Using the ANSI-JOIN syntax gives you an important advantage: Because the join logic is cleanly separated from the filtering criteria, you can understand the query logic more quickly.
    The SQL Server old-style JOIN executes the filtering conditions before executing the joins, whereas the ANSI-style JOIN reverses this procedure (join logic precedes filtering).
    Perhaps the most compelling argument for switching to the ANSI-style JOIN is that Microsoft has explicitly stated that SQL Server will not support the old-style OUTER JOIN syntax indefinitely. Another important consideration is that the ANSI-style JOIN supports query constructions that the old-style JOIN syntax does not support.
  4. What is derived table?
    Derived tables are SELECT statements in the FROM clause referred to by an alias or a user-specified name. The result set of the SELECT in the FROM clause forms a table used by the outer SELECT statement. For example, this SELECT uses a derived table to find if any store carries all book titles in the pubs database:
    SELECT ST.stor_id, ST.stor_name
    FROM stores AS ST,
    (SELECT stor_id, COUNT(DISTINCT title_id) AS title_count
    FROM sales
    GROUP BY stor_id
    ) AS SA
    WHERE ST.stor_id = SA.stor_id
    AND SA.title_count = (SELECT COUNT(*) FROM titles)

 locks

  1. What are locks?
    Microsoft® SQL Server™ 2000 uses locking to ensure transactional integrity and database consistency. Locking prevents users from reading data being changed by other users, and prevents multiple users from changing the same data at the same time. If locking is not used, data within the database may become logically incorrect, and queries executed against that data may produce unexpected results.
  2. What are the different types of locks?
    SQL Server uses these resource lock modes.

Lock mode

Description

Shared (S)

Used for operations that do not change or update data (read-only operations), such as a SELECT statement.

Update (U)

Used on resources that can be updated. Prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.

Exclusive (X)

Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time.

Intent

Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).

Schema

Used when an operation dependent on the schema of a table is executing. The types of schema locks are: schema modification (Sch-M) and schema stability (Sch-S).

Bulk Update (BU)

Used when bulk-copying data into a table and the TABLOCK hint is specified.

  1. What is a dead lock? Give a practical sample? How you can minimize the deadlock situation? What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
    Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process.
    A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. (A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.)
  2. What is isolation level?
    An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. A lower isolation level increases concurrency, but at the expense of data correctness. Conversely, a higher isolation level ensures that data is correct, but can affect concurrency negatively. The isolation level required by an application determines the locking behavior SQL Server uses.
    SQL-92 defines the following isolation levels, all of which are supported by SQL Server:
    • Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
    • Read committed (SQL Server default level).
    • Repeatable read.
    • Serializable (the highest level, where transactions are completely isolated from one another).

Isolation level

Dirty read

Nonrepeatable read

Phantom

Read uncommitted

Yes

Yes

Yes

Read committed

No

Yes

Yes

Repeatable read

No

No

Yes

Serializable

No

No

No

  1. Uncommitted Dependency (Dirty Read) – Uncommitted dependency occurs when a second transaction selects a row that is being updated by another transaction. The second transaction is reading data that has not been committed yet and may be changed by the transaction updating the row. For example, an editor is making changes to an electronic document. During the changes, a second editor takes a copy of the document that includes all the changes made so far, and distributes the document to the intended audience.
    Inconsistent Analysis (Nonrepeatable Read) Inconsistent analysis occurs when a second transaction accesses the same row several times and reads different data each time. Inconsistent analysis is similar to uncommitted dependency in that another transaction is changing the data that a second transaction is reading. However, in inconsistent analysis, the data read by the second transaction was committed by the transaction that made the change. Also, inconsistent analysis involves multiple reads (two or more) of the same row and each time the information is changed by another transaction; thus, the term nonrepeatable read. For example, an editor reads the same document twice, but between each reading, the writer rewrites the document. When the editor reads the document for the second time, it has changed.
    Phantom Reads Phantom reads occur when an insert or delete action is performed against a row that belongs to a range of rows being read by a transaction. The transaction's first read of the range of rows shows a row that no longer exists in the second or succeeding read, as a result of a deletion by a different transaction. Similarly, as the result of an insert by a different transaction, the transaction's second or succeeding read shows a row that did not exist in the original read. For example, an editor makes changes to a document submitted by a writer, but when the changes are incorporated into the master copy of the document by the production department, they find that new unedited material has been added to the document by the author. This problem could be avoided if no one could add new material to the document until the editor and production department finish working with the original document.
  2. nolock? What is the difference between the REPEATABLE READ and SERIALIZE isolation levels?
    Locking Hints –
    A range of table-level locking hints can be specified using the SELECT, INSERT, UPDATE, and DELETE statements to direct Microsoft® SQL Server 2000 to the type of locks to be used. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.

Locking hint

Description

HOLDLOCK

Hold a shared lock until completion of the transaction instead of releasing the lock as soon as the required table, row, or data page is no longer required. HOLDLOCK is equivalent to SERIALIZABLE.

NOLOCK

Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement.

PAGLOCK

Use page locks where a single table lock would usually be taken.

READCOMMITTED

Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server 2000 operates at this isolation level.

READPAST

Skip locked rows. This option causes a transaction to skip rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement.

READUNCOMMITTED

Equivalent to NOLOCK.

REPEATABLEREAD

Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level.

ROWLOCK

Use row-level locks instead of the coarser-grained page- and table-level locks.

SERIALIZABLE

Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK.

TABLOCK

Use a table lock instead of the finer-grained row- or page-level locks. SQL Server holds this lock until the end of the statement. However, if you also specify HOLDLOCK, the lock is held until the end of the transaction.

TABLOCKX

Use an exclusive lock on a table. This lock prevents others from reading or updating the table and is held until the end of the statement or transaction.

UPDLOCK

Use update locks instead of shared locks while reading a table, and hold locks until the end of the statement or transaction. UPDLOCK has the advantage of allowing you to read data (without blocking other readers) and update it later with the assurance that the data has not changed since you last read it.

XLOCK

Use an exclusive lock that will be held until the end of the transaction on all data processed by the statement. This lock can be specified with either PAGLOCK or TABLOCK, in which case the exclusive lock applies to the appropriate level of granularity.

  1. For example, if the transaction isolation level is set to SERIALIZABLE, and the table-level locking hint NOLOCK is used with the SELECT statement, key-range locks typically used to maintain serializable transactions are not taken.
    USE pubs
    GO
    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
    GO
    BEGIN TRANSACTION
    SELECT au_lname FROM authors WITH (NOLOCK)
    GO
  2. What is escalation of locks?
    Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it's dynamically managed by SQL Server.

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