SQL stands for Structured Query Language. It's used with all kinds of relational databases. This guide provides
a basic, high level description of the syntax for SQL statements. SQL is an international standard (ISO), but you will find many differences between implementations. This guide uses MySQL as an example. If you use one of the many other Relational Database Managers (DBMS) you’ll need to check the manual for that DBMS if needed. This is used to select the database containing the tables for your SQL statements: The Select part is normally used to determine which columns of the data you want to show in the results. There are also options you can use to show data that is not a table column. This example
shows two columns selected from the “student” table, and two calculated columns. The first of the calculated columns is a meaningless number, and the other is the system date. The WHERE clause is used to limit the number of rows returned. In this case all five of these will be used is a somewhat ridiculous Where clause. Compare this result to the above SQL statement to follow this logic. Rows will be presented that: The following example is similar,
but it further specifies that if any of the students have certain SAT scores (1000, 1400), they will not be presented: Order By gives us a way to sort the result set by one or more of the items in the SELECT section. Here is the same list as above, but sorted by the students Full
Name. The default sort order is ascending (ASC), but to sort in the opposite order (descending) you use DESC, as in the example below: Group By gives us a way to combine rows and aggregate data. The Having clause is like the above Where clause, except that it acts on the grouped
data. This data is from the campaign contributions data we’ve been using in some of these guides. This SQL statement answers the question: “which candidates recieved the largest number of contributions (not $ amount, but count (*)) in 2016, but only those who had more than 80 contributions?” Ordering this data set in a descending (DESC) order places the candidates with the largest number of contributions at the top of the list.
As with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide. I hope this at least gives you enough to get started. Please see the manual for your database manager and have fun trying different options yourself. Common SQL Interview QuestionsWhat is an inner join in SQL?This is the default type of join if no join is specified. It returns all rows in which there is at least one match in both tables.
What is a left join in SQL?A left join returns all rows from the left table, and the matched rows from the right table. Rows in the left table will be returned even if there was no match in the right table. The rows from the left table with no
match in the right table will have
What is a right join in SQL?A right join returns all rows from the right table, and the matched rows from the left table. Opposite of a left join, this will return all rows from the right table even where there is no match in the left table. Rows in the right table that have no match in the left table will have
What is a full join in SQL?A full join returns all rows for which there is a match in either of the tables. So if there are rows in the left table that do not have matches in the right table, those will be included. As well as if there are rows in the right table that do not have matches in the left table, those will be included.
What is the result of the following command?
Here it’ll be an error because we can’t perform a DML operation on a view. Can we perform a rollback after using ALTER command?No, because ALTER is a DDL command and Oracle server performs an automatic COMMIT when the DDL statements are executed. Which is the only constraint that enforces rules at column level?NOT NULL is the only constraint that works at the column level. What are the pseudocolumns in SQL? Give some examples?A pseudocolumn is a function which returns a system generated value. The reason it is known as so because a pseudocolumn is an Oracle assigned value used in the same context as an Oracle database column but not stored on disk.
Create a user my723acct with password kmd26pt. Use the user data and temporary data tablespaces provided by PO8 and provide to this user 10M of storage space in user data and 5M of storage space in temporary_data.
Create the role role tables and_views.
Grant to the role of the previous question the privileges to connect to the database and the privileges to create tables and views. The privilege to connect to the database is CREATE SESSION The privilege to create table is CREATE TABLE The privilege to create view is CREATE VIEW
Grant the previous role in the question to the users anny and rita
Create a user my723acct with password kmd26pt. Use the user data and temporary data tablespaces provided by PO8 and provide to this user 10M of storage space in user data and 5M of storage space in temporary_data.
Create the role role tables and_views.
Grant to the role of the previous question the privileges to connect to the database and the privileges to create tables and views. The privilege to connect to the database is CREATE SESSION The privilege to create table is CREATE TABLE The privilege to create view is CREATE VIEW.
Grant the previous role in the question to the users anny and rita
Write a command to change the password of the user rita from abcd to dfgh
The users rita and anny do not have SELECT privileges on the table INVENTORY that was created by SCOTT. Write a command to allow SCOTT to grant the users SELECT priviliges on these tables.
User rita has been transferred and no longer needs the privilege that was granted to her through the role role tables and_views. Write a command to remove her from her previous given priviliges except that she still could connect to the database.
The user rita who was transferred is now moving to another company. Since the objects that she created is of no longer use, write a commmand to remove this user and all her objects. Here CASCADE option is necessary to remove all the objects of the user in the database.
The user rita who was transferred is now moving to another company. Since the objects that she created is of no longer use, write a command to remove this user and all her objects. Here CASCADE option is necessary to remove all the objects of the user in the database.
Write SQL query to find the nth highest salary from table.
SQL Create View StatementWhat is a View?A View is a database object that presents data existing in one or more tables. Views are used in a similar way to tables, but they don’t contain any data. They just “point” to the data that exists elsewhere (tables or views, for example). Why do we like them?
Important Safety Tips
Syntax of the Create View Statement (MySQL)
This guide will cover this part of of the statement…
Sample View creation from the student tablesNotes:
Sample of using a View to combine data from more than one tableA Student demographics table was added to the database to demonstrate this usage. This view will combine these tables. Notes:
Guide to the SQL Between OperatorThe BETWEEN Operator is useful because of the SQL Query Optimizer. Although BETWEEN is functionally the same as: x <= element <= y, the SQL Query Optimizer will recognize this command faster, and has optimized code for running it. This operator is used in a WHERE clause or in a GROUP BY HAVING clause. Rows are selected that have a value greater than the minimum value and less than the maximum value. It’s important to keep in mind that the values entered in the command are excluded from the result. We get just what is between them. Here is the syntax for using the function in a WHERE Clause:
Here is an example using the student table and the WHERE clause:
Here is an example using the campaign funds table and the having clause. This will return rows where the sum of the donations for a candidate are between $3 Million and $18 Million based on the HAVING clause in the GROUP BY part of the statement. More on aggregation in that guide.
SQL Create Table Statement ExampleA table is a group of data stored in a database. To create a table in a database you use the
Here’s an example creating a table named Person:
In the example above, each Person has a Name, a Date of Birth and a Gender. The Id column is the key that identifies one person in the table. You use the keyword A column can be A guide to the SQL Insert QueryInsert queries are a way to insert data into a table. Let’s say we have created a table using
example_table
Now to add some data to this table , we’ll use INSERT in the following way:
example_table
Even the following will work, but it’s always a good practice to specify which data is going into which column.
example_table
A guide to the SQL AND operatorAND is used in a WHERE clause or a GROUP BY HAVING clause to limit the rows returned from the executed statement. Use AND when it’s required to have more than one condition met. We’ll use the student table to present examples. Here’s the student table without a WHERE clause:
Now the WHERE clause is added to display only programming students:
Now the WHERE clause is updated with AND to show results for programming students that also have a SAT score greater than 800:
This is a more complex example from the campaign contributions table. This example has a GROUP BY clause with HAVING clause using an AND to restrict the returned records to candidates from 2016 with contributions between $3 Million and $18 Million in total.
How to use the SQL Order By KeywordOrder By (ASC, DESC)ORDER BY gives us a way to SORT the result set by one or more of the items in the SELECT section. Here is an SQL sorting the students by FullName in descending order. The default sort order is ascending (ASC) but to sort in the opposite order (descending) you use DESC.
Here is the UN-ORDERED, current, full student list to compare to the above.
As with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide. I hope this at least gives you enough to get started. Please see the manual for your database manager and have fun trying different options yourself. Learn to code for free. freeCodeCamp's open source curriculum has helped more than 40,000 people get jobs as developers. Get started When a reward is given every time a target behavior occurs it is known as a N?In continuous reinforcement, the desired behavior is reinforced every single time it occurs. 1 This schedule is best used during the initial stages of learning to create a strong association between the behavior and response.
What type of team has a well defined purpose?High-performing teams are aligned in their focus, purpose, and priorities. They set team and individual goals that support this shared vision so that their work drives achievement. Goals are not only aligned, but they are clearly defined so everyone knows exactly what they need to do and how to get there.
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