Sample Sql Queries For Practice – Below are examples of some very important complex SQL queries with answers. I have tried to explain each question in detail so that everyone can understand how to apply it step by step. In SQL and PL SQL interview we need to know the basic and important complex sql queries in detail. These are important complex SQL queries:
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Select * from Emploee e where rownum <=5 select union * (select * from Emploee and order by rowid desk) where rownum <=5;
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Select * from ( a.*, rownum rnum from ( YOUR_QUERY_GOES_HERE — including order by ) a where rownum = N_ROWS
Select salary distinct from employee a where 3 >= (select count(distinct salary) from employee b where a.salary <= b.salary) order by a.salary desc;
Select * from (select rnom as rno, E.* from employee) where Mod(rno, 2)=0; Learn the SQL Server course here: SQL Server Training
FROM * (SELECT Dense_Rank() (ORDER BY salary) FROM Rnk, E.* employee FROM E) WHERE Rnk=3;
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We cannot use a double table to display the above results. Use any table to display the result. I use a student table.
Hint: The user should be familiar with the concept of sequential queries. Click here to get concept of series questions
I hope this article titled “Examples of Complex SQL Queries” will be useful for all programmers. This article gives you an idea of examples of complex SQL queries and will be useful for all programmers.
To_char function is used to convert time to characters and ssss is used to convert time to seconds.
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Basic SQL queries Complex View Difference between Simple View and Complex View Equi Join Inner Join Logical Table in SQL Non Equi Join Oracle Outer Join PLSQL Postgres Self Join Simple View SQL View Joins Views in SQL This article is about writing SQL queries. Covers some best practices for data analysts and scientists. Most of our discussion will be about SQL in general, but we’ll add some notes on special features to make writing SQL easy.
The standard caveat about premature optimization applies here. Avoid customizing an SQL query unless you know your query will return the data you’re looking for. And even then, it’s best to optimize your query only if it’s run frequently (for example, by enabling a popular dashboard), or if the query traverses a large number of rows. In general, prioritize correctness (does the query return the desired results) and readability (others can easily understand and modify the code) before worrying about performance.
We may already be starting to optimize here, but the goal should be to tell the database to scan the minimum number of values necessary to produce results.
Part of the beauty of SQL is its declarative nature. Instead of telling the database how to get the records, you only need to tell the database what records you want, and the database has to figure out the most efficient way to get that information. So a lot of the query performance advice is about showing people how to use the tools in SQL to better define their needs.
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We’ll go over the general sequence of query execution and add tips for narrowing the search space. Then we’ll talk about three essential tools to add to your utility belt: INDEX, EXPLANATION, and Q.
Familiarize yourself with your data before you write a single line of code by examining the metadata to ensure that the column actually contains the data you expect. The SQL editor has a handy Data Reference tab (accessible via the book icon) where you can browse the tables in your database and view their columns and relationships (Figure 1):
Gives you many different ways to explore your data: you can write queries using the X-ray table, query builder and notepad editor, convert a saved query to SQL code, or you can build from an existing parent query. We cover this in other articles; For now, let’s look at a typical query workflow.
Before we get into individual tips for writing SQL code, it’s important to understand how the database will execute your query. This is different from the reading order (left-to-right, top-to-bottom) that you use to write your query. Query optimizers may change the order of the list below, but it’s good to keep this general SQL query life cycle in mind when writing SQL. We’ll use the order of execution to group tips for writing good SQL in the future.
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The rule of thumb here is that the sooner you can remove data from this list, the better.
And, of course, there will always be times when the query optimizer will generate a different query plan for your particular database, so don’t stop there.
The following tips are guidelines, not rules, intended to keep you out of trouble. Each database handles SQL differently, has a slightly different feature set, and uses different approaches to query optimization. And that’s before we compare traditional transactional databases to analytical databases that use columnar storage formats, which have very different performance characteristics.
Help people (including yourself three months from now) by adding comments that explain different parts of the code. The most important thing to capture here is the “why.” For example, the following code clearly filters orders
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Than 10, but the reason for this is that the first 10 orders are used for testing.
The catch here is that you add a bit of maintenance overhead: if you change the code, you need to make sure the comment stays current and relevant. But that’s a small price to pay for readable code.
When querying multiple tables, use aliases and include those aliases in your select statement so the database (and your reader) doesn’t have to figure out which column belongs to which table. Note: If you have columns with the same name in multiple tables, you’ll need to explicitly reference them by table name or alias.
This is a simple example, but as the number of tables and columns in your query increases, your readers won’t have to figure out which column is in which table. This query and yours can be broken if you join a table with an ambiguous column name (for example, both tables contain a field named
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Note that field filters are not compatible with table aliases, so you will need to remove aliases when connecting filter widgets to your field filters.
To filter out extra rows, so you don’t need to calculate these values first. After removing only the irrelevant rows and collecting and grouping those rows, you should add
The clause can actually slow down your query because the function makes the query unsorted (that is, prevents the database from using the index to speed up the query). Instead of using an index to jump to the corresponding rows, a function for a column forces the database to run the function for each row in the table.
There is also a function so don’t try to combine rows to filter multiple columns. Instead, prefer a few terms:
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You can use indexes (if they exist for the field) as long as you avoid adding a wildcard operator to the search term,
Using wildcards for searches can be expensive. It is recommended to add a wildcard character at the end of the line. Adding a wildcard character to a string can cause a full table scan.
Columns are in order of decreasing power. That is, first group by columns with more unique values (such as ID or phone number) before grouping by columns with less distinct values (such as state or gender).
This section is for the database administrators in the crowd (and is too big a topic to fit in this article). One of the most common things people run into when they run into database query performance issues is a lack of adequate indexing.
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Which columns you should index usually depends on the columns you’re filtering on (ie, which columns are usually your
If you find that you always filter on a common set of columns, you should consider listing those columns.
Indexing foreign key columns and frequently queried columns can significantly reduce query time. Here is an example of creating an index:
There are different types of indexes, the most common type of index uses B-trees to speed up searches. Check out our article on creating dashboards quickly and refer to your database’s documentation for creating indexes.
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For particularly large datasets or one-dimensional datasets where certain ranges of values appear more often, consider creating an index.
Clause to limit the number of indexed rows. Partial indexes can also be useful for date ranges, for example if you only want to display data from the last week.
For columns that are normally connected
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