sql nested query example

respectively calculated from the other two tables.So the left link is set to six layers... A view does not improve the efficiency. It is only used to control permissions or query formats. Do not use left Outer JoinLow EfficiencyWrite it like this.Sele AA. * from a AA,(Select a_idn, sum (Working Hours) from B group by a_idn) BB where AA. IDN = BB. a_idn Sele AA. * from a AA,(Select a_idn, sum (Working Hours) from B group by a_i

. For example, when multiple indexes are used, the SELECT query may run faster. However, the speed of DML (insert, update, and delete) operations will be significantly slowed down because more indexes must be maintained for each operation. Therefore, if your query mainly contains select statements, it is very helpful to use more indexes. If your applicationProgra

. customer_id, C. ID, C. NameFrom orders o left Outer Join MERs C on C. ID = O. customer_idUnionSelect O. ID, O. order_number, O. customer_id, C. ID, C. NameFrom orders o right Outer Join MERs C on C. ID = O. customer_id; The query results for statement 9 and 10 are the same, as shown below: 4. Union join ):This is a rare connection method. Oracle and MySQL do not support this function. The function is to find all rows with the difference between the

. customer_id, c. id, c. nameFrom orders o left outer join MERs c on c. id = o. customer_idUnionSelect o. id, o. order_number, o. customer_id, c. id, c. nameFrom orders o right outer join MERs c on c. id = o. customer_id; The query results for statement 9 and 10 are the same, as shown below: 4. union join ):This is a rare connection method. Oracle and mysql do not support this function. The function is to find all rows with the difference between the

row of this sys.dm_os_buffer_descriptors dynamic management view (DMV) represents each data page held by the current memory, and you can use this script to see how much space each database occupies in the data cache: 1 SELECT Count(*)*8/1024x768 as 'Cached Size (MB)' 2, Casedatabase_id3 when 32767 Then 'Resourcedb' 4 ELSE db_name(database_id)5 END as 'Database' 6 fromsys.dm_os_buffer_descriptors7 GROUP by db_name(database_id), database_id8 ORDER by 'Cached Size (

In our website design process, multi-condition queries are often used. the source code of this article is an example of a second-hand house query. In this example, we need to obtain the information required by the customer through multiple conditions, such as geographical location, property type, house price, house area, and information release date. The followin

In our site design process, often use multi-conditional query, the source of this article is a second-hand housing query example. In this case, we are going to be able to find the information that we need through a number of conditions, such as location, property type, house price, housing area and information release date. The following is the implementation pro

PHP's YII framework uses database configuration and SQL Operations Examples tutorial, yii example Tutorial Database access (DAO)YII contains a data access layer (DAO) built on top of the PHP PDO. DAO provides a unified set of APIs for different databases. Where ActiveRecord provides interaction between the database and the model (M,model in MVC), QueryBuilder is used to create dynamic

Gender column. There are only two gender values: male and female, when we have a gender field in our query condition, at least half of the data will be filtered out, which can greatly narrow the scope of the data to be retrieved. How can this be useless? (In fact, this is where I used to be confused.) We have a correct understanding. For example, on the Gender column of the Users table, create the index IX

, $ params );FindAllByPk ($ id, "name like ': name' and age =: age", array (': name' => $ name, 'age' => $ age ));This method queries a set based on the primary key. Multiple primary keys can be used, for example:FindAllByPk (array (1, 2 )); 3. $ admin = Admin: model ()-> findAllByAttributes ($ attributes, $ condition, $ params );This method is used to query a set based on conditions. Multiple conditions can be used t

loop statement. It is worth noting that when the while loop is nested, The continue keywords and break keywords only apply to the while loop in which they reside, and do not apply to the external while loop. A simple example is as follows: from 1 to 10, when the cycle is 7, the cycle ends and continues. When the cycle is 8, the cycle jumps out. If. Else keyword The IF... else keyword implements

, simple and forced. When parameterization settings are set to simple, only simple T-SQL statements are parameterized. To introduce this, see the following command: Select sum (linetotal) as linetotalFrom adventureworks. Sales. salesorderheader HJoin adventureworks. Sales. salesorderdetail D on D. salesorderid = H. salesorderidWhere H. salesorderid = 56000 This query is similar to my

whether the index is beneficial to the query. There are many articles that read the query plan, including MSDN Library a few in the. We do not intend to extend or replace them. In fact, we will provide links/references for many of them at this level . A good place to start is to display a graphical execution plan. (http://msdn.microsoft.com/en-us/library/ms178071.aspx). Other useful resources include the

Now the development system involves querying by page. when doing this, I am thinking, whether or not the query SQL statement can be automatically encapsulated by parameters into the ability to count the data size of the query statement that is not queried by page. for example, the general practice is: 1. first

of prefetch_related () is to query each table separately and then use Python to process the relationship between them. Continue with the example above. If we want to obtain all the cities that James has been to, use prefetch_related () to do this: >>> Zhangs = Person. objects. prefetch_related ('visitation '). get (firstname = u "Zhang", lastname = u "3") >>> for city in zhangs. visitation. all ():... prin

number of records read. Naively, I once thought that the number of logical reads in the query plan is the number of records read. However, according to our query, 4.1 full table scans return 830 rows of data, why are there only 22 logical reads, and 4.5 queries also return 830 rows of data. Why are 1724 logical reads? If one read is performed, 22 logical reads can return up to 22 rows of data, if the numb