Los mejores ejemplos de SQL

SQL son las siglas de Structured Query Language. Se usa con todo tipo de bases de datos relacionales.

Ejemplo de sintaxis SQL básica

Esta guía proporciona una descripción básica de alto nivel de la sintaxis de las sentencias SQL.

SQL es un estándar internacional (ISO), pero encontrará muchas diferencias entre las implementaciones. Esta guía usa MySQL como ejemplo. Si utiliza uno de los muchos otros administradores de bases de datos relacionales (DBMS), deberá consultar el manual de ese DBMS si es necesario.

Que cubriremos

  • Usar (establece qué base de datos usará la declaración)
  • Cláusulas Select y From
  • Where Cláusula (y / o, IN, Between, LIKE)
  • Ordenar por (ASC, DESC)
  • Agrupar por y tener

Como usar esto

Esto se usa para seleccionar la base de datos que contiene las tablas para sus declaraciones SQL:

use fcc_sql_guides_database; -- select the guide sample database 

Cláusulas Select y From

La parte Seleccionar se usa normalmente para determinar qué columnas de los datos desea mostrar en los resultados. También hay opciones que puede utilizar para mostrar datos que no son una columna de tabla.

Este ejemplo muestra dos columnas seleccionadas de la tabla "estudiante" y dos columnas calculadas. La primera de las columnas calculadas es un número sin sentido y la otra es la fecha del sistema.

 select studentID, FullName, 3+2 as five, now() as currentDate from student; 
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Where Cláusula (y / o, IN, Between y LIKE)

La cláusula WHERE se usa para limitar el número de filas devueltas.

En este caso, se utilizarán los cinco, es una cláusula Where algo ridícula.

Compare este resultado con la declaración SQL anterior para seguir esta lógica.

Se presentarán filas que:

  • Tener identificaciones de estudiante entre 1 y 5 (inclusive)
  • o studentID = 8
  • o tener "Maxmimo" en el nombre

El siguiente ejemplo es similar, pero especifica además que si alguno de los estudiantes tiene ciertos puntajes en el SAT (1000, 1400), no se presentarán:

 select studentID, FullName, sat_score, recordUpdated from student where ( studentID between 1 and 5 or studentID = 8 or FullName like '%Maximo%' ) and sat_score NOT in (1000, 1400); 
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Ordenar por (ASC, DESC)

Ordenar por nos ofrece una forma de ordenar el conjunto de resultados por uno o más de los elementos de la sección SELECCIONAR. Esta es la misma lista que la anterior, pero ordenada por el nombre completo del estudiante. El orden de clasificación predeterminado es ascendente (ASC), pero para ordenar en el orden opuesto (descendente) se usa DESC, como en el ejemplo siguiente:

 select studentID, FullName, sat_score from student where (studentID between 1 and 5 -- inclusive or studentID = 8 or FullName like '%Maximo%') and sat_score NOT in (1000, 1400) order by FullName DESC; 
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Agrupar por y tener

Agrupar por nos brinda una forma de combinar filas y agregar datos. La cláusula Have es como la cláusula Where anterior, excepto que actúa sobre los datos agrupados.

Estos datos provienen de los datos de contribuciones de campaña que hemos estado usando en algunas de estas guías.

Esta declaración SQL responde a la pregunta: "¿Qué candidatos recibieron la mayor cantidad de contribuciones (no el monto en dólares, pero cuentan (*)) en 2016, pero solo aquellos que tuvieron más de 80 contribuciones?"

Ordenar este conjunto de datos en orden descendente (DESC) coloca a los candidatos con el mayor número de contribuciones al principio de la lista.

 select Candidate, Election_year, sum(Total_$), count(*) from combined_party_data where Election_year = 2016 group by Candidate, Election_year having count(*) > 80 order by count(*) DESC; 
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Al igual que con todas estas cosas de SQL, hay MUCHO MÁS que lo que se encuentra en esta guía introductoria. Espero que esto al menos te dé lo suficiente para comenzar. Consulte el manual de su administrador de base de datos y diviértase probando diferentes opciones usted mismo.

Preguntas comunes de la entrevista SQL

¿Qué es una combinación interna en SQL?

Este es el tipo de combinación predeterminado si no se especifica ninguna combinación. Devuelve todas las filas en las que hay al menos una coincidencia en ambas tablas.

SELECT * FROM A x JOIN B y ON y.aId = x.Id 

¿Qué es una combinación izquierda en SQL?

Una combinación izquierda devuelve todas las filas de la tabla izquierda y las filas coincidentes de la tabla derecha. Las filas de la tabla de la izquierda se devolverán incluso si no hubo coincidencia en la tabla de la derecha. Las filas de la tabla de la izquierda que no coincidan en la tabla de la derecha tendrán los nullvalores de la tabla de la derecha.

SELECT * FROM A x LEFT JOIN B y ON y.aId = x.Id 

¿Qué es una combinación derecha en SQL?

Una combinación derecha devuelve todas las filas de la tabla derecha y las filas coincidentes de la tabla izquierda. Al contrario de una combinación de la izquierda, esto devolverá todas las filas de la tabla de la derecha incluso cuando no haya coincidencias en la tabla de la izquierda. Las filas de la tabla de la derecha que no coinciden en la tabla de la izquierda tendrán nullvalores para las columnas de la tabla de la izquierda.

SELECT * FROM A x RIGHT JOIN B y ON y.aId = x.Id 

¿Qué es una combinación completa en 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.

SELECT Customers.CustomerName, Orders.OrderID FROM Customers FULL OUTER JOIN Orders ON Customers.CustomerID=Orders.CustomerID ORDER BY Customers.CustomerName 

What is the result of the following command?

 DROP VIEW view_name 

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.

 ROWNUM, ROWID, USER, CURRVAL, NEXTVAL etc. 

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 USER my723acct IDENTIFIED BY kmd26pt DEFAULT TABLESPACE user_data TEMPORARY TABLESPACE temporary_data QUOTA 10M on user_data QUOTA 5M on temporary_data 

Create the role role tables and_views.

 CREATE 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 Create session, create table, create view TO role_tables_and_views 

Grant the previous role in the question to the users anny and rita

 GRANT role_tables_and_views TO anny, 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 USER my723acct IDENTIFIED BY kmd26pt DEFAULT TABLESPACE user_data TEMPORARY TABLESPACE temporary_data QUOTA 10M on user_data QUOTA 5M on temporary_data 

Create the role role tables and_views.

 CREATE 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 Create session, create table, create view TO role_tables_and_views 

Grant the previous role in the question to the users anny and rita

 GRANT role_tables_and_views TO anny, rita 

Write a command to change the password of the user rita from abcd to dfgh

 ALTER USER rita IDENTIFIED BY 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.

 GRANT select ON inventory TO rita, anny 

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.

 REVOKE select ON scott.inventory FROM rita REVOKE create table, create view FROM rita 

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.

 DROP USER rita CASCADE ### 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. ``` sql REVOKE select ON scott.inventory FROM rita REVOKE create table, create view FROM rita 

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.

 DROP USER rita CASCADE 

Write SQL query to find the nth highest salary from table.

 SELECT TOP 1 Salary FROM ( SELECT DISTINCT TOP N Salary FROM Employee ORDER BY Salary DESC ) ORDER BY Salary ASC

SQL Create View Statement

What 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?

  • Views are a way to limit the data presented. For example, the human resources department data filtered to only present non-sensitve information. Sensitive information in this case could be social security numbers, sex of employee, payrate, home address, etc.
  • Complex data across more than one table can be combined into a single “view.” This can make life easier for your business analysts and programmers.

Important Safety Tips

  • Views are managed by the system. When data in the related tables are changed, added, or updated, the View is updated by the system. We want to use these only when needed to manage use of system resources.
  • In MySQL, changes to the table design (that is, new or dropped columns) made AFTER a view is created are not updated in the view itself. The view would have to be updated or recreated.
  • Views are one of the four standard database object types. The others are tables, stored procedures, and functions.
  • Views can usually be treated as you would a table, but updates are limited or not available when the view contains more than one table.
  • There are many other details about views that are beyond the scope of this introductory guide. Spend time with your database managers manual and have fun with this powerful SQL object.

Syntax of the Create View Statement (MySQL)

CREATE [OR REPLACE] [ALGORITHM = UNDEFINED ] [DEFINER =  user ] [SQL SECURITY  DEFINER ] VIEW view_name [(column_list)] AS select_statement [WITH [CASCADED | LOCAL] CHECK OPTION] 

This guide will cover this part of of the statement…

CREATE VIEW view_name [(column_list)] AS select_statement 

Sample View creation from the student tables

Notes:

  • The name of the view has a “v” at the end. It’s recommended that the view name indicate that it’s a view in some way to make life easier for programmers and database administrators. Your IT shop should have its own rules on naming objects.
  • The columns in the view are limited by the SELECT and the rows of data by the WHERE clause.
  • the ”`” character around the view names is required because of the ”-” in the names. MySQL reports an error without them.
create view `programming-students-v` as select FullName, programOfStudy from student where programOfStudy = 'Programming'; select * from `programming-students-v`; 

Sample of using a View to combine data from more than one table

A Student demographics table was added to the database to demonstrate this usage. This view will combine these tables.

Notes:

  • To “join” tables, the tables must have fields in common (usually primary keys) that uniquely identity each row. In this case it’s the student ID. (More on this in the SQL Joins guide.)
  • Notice the “alias” given to each table (“s” for student and “sc” for student contact). This is a tool to shorten the table names and make it easier to identify which table is being used. It’s easier than typing long table names repeatedly. In this example, it was required because studentID is the same column name in both tables, and the system would present an “ambiguous column name error” without specifying which table to use.

Guide to the SQL Between Operator

The 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:

select field1, testField from table1 where testField between min and max 

Here is an example using the student table and the WHERE clause:

-- no WHERE clause select studentID, FullName, studentID from student; -- WHERE clause with between select studentID, FullName, studentID from student where studentID between 2 and 7; 
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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.

select Candidate, Office_Sought, Election_Year, format(sum(Total_$),2) from combined_party_data where Election_Year = 2016 group by Candidate, Office_Sought, Election_Year having sum(Total_$) between 3000000 and 18000000 order by sum(Total_$) desc; 
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SQL Create Table Statement Example

A table is a group of data stored in a database.

To create a table in a database you use the CREATE TABLE statement. You give a name to the table and a list of columns with its datatypes.

CREATE TABLE TABLENAME(Attribute1 Datatype, Attribute2 Datatype,........); 

Here’s an example creating a table named Person:

CREATE TABLE Person( Id int not null, Name varchar not null, DateOfBirth date not null, Gender bit not null, PRIMARY KEY( Id ) ); 

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 PRIMARY KEY to configure one or more columns as a primary key.

A column can be not null or null indicating whether it is mandatory or not.

A guide to the SQL Insert Query

Insert queries are a way to insert data into a table. Let’s say we have created a table using

CREATE TABLE example_table ( name varchar(255), age int)

example_table

Name|Age --- | --- 

Now to add some data to this table , we’ll use  INSERT  in the following way:

INSERT INTO example_table (column1,column2) VALUES ("Andrew",23)

example_table

Name|Age --- | --- Andrew|23

Even the following will work, but it’s always a good practice to specify which data is going into which column.

INSERT INTO table_name VALUES ("John", 28)

example_table

Name|Age --- | --- Andrew|23 John|28

A guide to the SQL AND operator

AND 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:

select * from student; 
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Now the WHERE clause is added to display only programming students:

select * from student where programOfStudy = 'Programming'; 
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Now the WHERE clause is updated with AND to show results for programming students that also have a SAT score greater than 800:

select * from student where programOfStudy = 'Programming' and sat_score > 800; 
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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.

select Candidate, Office_Sought, Election_Year, FORMAT(sum(Total_$),2) from combined_party_data where Office_Sought = 'PRESIDENT / VICE PRESIDENT' group by Candidate, Office_Sought, Election_Year having Election_Year = 2016 and sum(Total_$) between 3000000 and 18000000 order by sum(Total_$) desc; 
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How to use the SQL Order By Keyword

Order 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.

SELECT studentID, FullName, sat_score FROM student ORDER BY FullName DESC; 
+-----------+------------------------+-----------+ | studentID | FullName | sat_score | +-----------+------------------------+-----------+ | 2 | Teri Gutierrez | 800 | | 3 | Spencer Pautier | 1000 | | 6 | Sophie Freeman | 1200 | | 9 | Raymond F. Boyce | 2400 | | 1 | Monique Davis | 400 | | 4 | Louis Ramsey | 1200 | | 7 | Edgar Frank "Ted" Codd | 2400 | | 8 | Donald D. Chamberlin | 2400 | | 5 | Alvin Greene | 1200 | +-----------+------------------------+-----------+ 9 rows in set (0.00 sec) 

Here is the UN-ORDERED, current, full student list to compare to the above.

SELECT studentID, FullName, sat_score, rcd_updated FROM student; 
+-----------+------------------------+-----------+---------------------+ | studentID | FullName | sat_score | rcd_updated | +-----------+------------------------+-----------+---------------------+ | 1 | Monique Davis | 400 | 2017-08-16 15:34:50 | | 2 | Teri Gutierrez | 800 | 2017-08-16 15:34:50 | | 3 | Spencer Pautier | 1000 | 2017-08-16 15:34:50 | | 4 | Louis Ramsey | 1200 | 2017-08-16 15:34:50 | | 5 | Alvin Greene | 1200 | 2017-08-16 15:34:50 | | 6 | Sophie Freeman | 1200 | 2017-08-16 15:34:50 | | 7 | Edgar Frank "Ted" Codd | 2400 | 2017-08-16 15:35:33 | | 8 | Donald D. Chamberlin | 2400 | 2017-08-16 15:35:33 | | 9 | Raymond F. Boyce | 2400 | 2017-08-16 15:35:33 | +-----------+------------------------+-----------+---------------------+ 9 rows in set (0.00 sec) 

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.

Consulte el manual de su administrador de base de datos y diviértase probando diferentes opciones usted mismo.