Sql complex queries examples pdf free download
Example: Updates the value and quantity values for an order with an id of in the orders table. Example: Updates the mileage and serviceDue values for a vehicle with an id of 45 in the cars table. Comments allow you to explain sections of your SQL statements, or to comment out code and prevent its execution.
Single line comments start with —. Any text after these 2 characters to the end of the line will be ignored. They stretch across multiple lines until the closing characters have been found.
When creating a new table or editing an existing one, you must specify the type of data that each column accepts. For size, the maximum number is 65 and the default is 10, whilst for d, the maximum number is 30 and the default is Example: Lists the number of users with active orders, but only include users with more than 3 active orders. In relational databases, there is a concept of primary and foreign keys. In SQL tables, these are included as constraints, where a table can have a primary key, a foreign key, or both.
A primary key allows each record in a table to be uniquely identified. There can only be one primary key per table, and you can assign this constraint to any single or combination of columns.
However, this means each value within this column s must be unique. This means the value increases automatically as new records are created. A foreign key can be applied to one column or many and is used to link 2 tables together in a relational database.
As seen in the diagram below, the table containing the foreign key is called the child key, whilst the table which contains the referenced key, or candidate key, is called the parent table.
Indexes are attributes that can be assigned to columns that are frequently searched against to make data retrieval a quicker and more efficient process. This is because when indexed columns are updated, the index itself must also be updated.
In SQL, a JOIN clause is used to return a results set which combines data from multiple tables, based on a common column which is featured in both of them.
In the following example, an inner join will be used to create a new unifying view combining the orders table and then 3 different tables. A view is essentially a SQL results set that get stored in the database under a label, so you can return to it later, without having to rerun the query. These are especially useful when you have a costly SQL query which may be needed a number of times, so instead of running it over and over to generate the same results set, you can just do it once and save it as a view.
This makes SQL a valuable language to know, as it allows you to create more complex, functional websites, and systems. Twitter Facebook.
Last updated: Aug 19, 5 Comments. For macOS and Linux users, Wine is first required as a prerequisite. This example query returns all countries with Queen Elizabeth II as their head of state. Cheat Sheet Keywords A collection of keywords used in SQL statements, a description, and where appropriate an example.
It can also be used to add and delete constraints in a table, as per the above. Example 1: Selects stock with a quantity between and Example: Removes the users table. Example: Lists any dealerships with a deal finance percentage less than Example: Selects data from the users table. Example: Adds a new vehicle. It also requires aggregation.
Its execution plan is similar in nature to the one we saw in Figure 4, but with additional operations and a higher cost, of 7. Listing 7 creates an indexed view, vSalesSummaryCustomerProduct , to help reduce the cost of this and similar queries. It is there for the internal maintenance of indexed views — it maintains a count of the rows per group in the indexed view. We can see that SQL Server has created a statistics object for this clustered index, as shown in Figure What is going on?
The difference lies in when and how SQL Server creates automatic statistics, and when it uses them. Simply put, if we do not use the WITH NOEXPAND hint when querying an indexed view, the query optimizer will not use statistics created on the indexed view and neither will it create or update statistics automatically i.
Without automatically created or updated statistics, there can be a slight or even drastic difference between the numbers of rows the optimizer estimates a query will return, and the actual number of rows returned. Pay attention to statistics warnings if you see them! What is the lesson to be learned here? Unfortunately, there may still be occasions when the query optimizer decides not to use an indexed view, even though it seems that it could satisfy a query.
In fact, SQL Server may refuse to use the clustered index or any non-clustered indexes on a view, even if we reference the view directly in the query. The execution plan, in Figure 13, shows that the plan references the underlying tables and ignores our view and its index. The query cost is. Now, the execution plan shows a clustered index seek, as shown in Figure 14, and the query cost is.
We can add non-clustered indexes to boost query performance. Once again, exercise care. SQL Server has to maintain every index we add to a view, every time someone updates one of the contributing base tables.
Indexed views work better for relatively static base tables. However, once the clustered index exists, we can easily add useful non-clustered indexes, just as we can for any normal table. When we run the query in Listing 12 again, the execution plan is as shown in Figure The query cost has gone down, to 1.
This time the optimizer chose a seek operation on the new non-clustered index, which is what we wanted. However, it also needed to perform a key lookup to return the additional columns contained in the SELECT clause but not included in the non-clustered index.
To make this index more effective, we can make it a covering index for this query by including all of the columns the query references, as shown in Listing When we run the query again, we see an optimized index seek on the non-clustered index.
We also see a significantly reduced query cost, down to. In SQL Server Standard Edition, we can still create indexed views, but the optimizer will not automatically consider its indexes when formulating an execution plan for a query; it will simply access all of the underlying tables. We also noted occasions, even when using SQL Server Enterprise Edition, when we may need to use this hint to get the plan we expect.
Bear in mind also that if you write queries in stored procedures, your applications, or reports that use WITH NOEXPAND and then drop the index on the view at a later point in time, the queries that reference that index will fail.
In short, the same proviso applies here as applies to the use of any table index , join, or query hints: use them cautiously and sparingly, and document them. Another, implied but not discussed directly, is that the indexed view definition can only reference tables, not other views.
SQL Server has to guarantee that it can return a consistent result set regardless of whether a query accesses a view or the underlying tables, so it will automatically maintain indexes in response to data modifications on the base tables. Rock the SQL! Do you need to combine two tables without a common column? Do you only use equals in your JOIN conditions? Learn how non-equi JOINs can assist with listing duplicates and joining tables based on a range of values. Have you ever wondered how to join three tables in SQL?
It's easy when you know the basics. Joining three tables can be as easy as joining two tables. It composes of data block number, file number and row number in the data block. Database links are created in order to form communication between various databases, or different environments like test, development and production.
The database links are read-only to access other information as well. It is a binary file. It records the structure of the database. It includes locations of several log files, names and timestamps. They can be stored in different locations to help in retrieval of information if one file gets corrupted. Consistency shows that data will not be reflected to other users until the data is commit, so that consistency is maintained.
Anonymous blocks are unnamed blocks that are not stored anywhere whilst sub-programs are compiled and stored in database. They are compiled at runtime. An autonomous transaction is an independent transaction of the main or parent transaction. It is not nested if it is started by another transaction.
Polymorphism is a feature of OOP. You're probably using Development databases maybe even one per developer Test databases Staging databases Production databases jOOQ lets you configure your environment easily, allowing you to rewrite generated SQL schema and table names.
More details can be seen in the manual: Schema and table rewriting at code-generation time Schema and table rewriting at runtime. Standardisation Each SQL dialect is different in various obvious and subtle ways. These differences include things like: SQL statements e. MERGE vs. END Pseudo elements e. Syntax elements e. It interfaces your two most important assets: Your data Your business logic You want to stay in full control of this interface, influencing SQL rendering, variable binding, query execution and other query operation lifecycles.
More details can be seen in the manual: Manual sections about ExecuteListeners Javadoc sections about ExecuteListeners. Procedures More and more SQL databases implement some sort of procedural language for in-database data processing and bulk operations. More details can be seen in the manual: Stored procedures and functions in jOOQ Generated artefacts from stored procedures and functions. Some of our customers More customers. Using jOOQ has been a joy, it's made life much easier for us.
Typesafety Examples SQL is a very type safe language. Predicates jOOQ type-checks simple comparison predicates and predicates with subqueries. Set Operations jOOQ type-checks degree and data types of union subselects.
Some more sophisticated examples show type-checks on row value expressions:. Hi there! That's why I created jOOQ.
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