习题七

          Current tables:

robots

id	name
1	Robot 2000
2	Champion Robot 2001
3	Dragon
4	Turbo Robot 2002
5	Super Robot 2003
6	Super Turbo Robot 2004
7	Not A Robot
8	Unreleased Turbo Robot 2111

           习题二十八、LIKE

use the LIKE command in order to search through text-based values. With LIKE, there are two special characters: % and _.

The percent sign (%) represents zero, one, or multiplecharacters. 

The underscore (_)represents one character. 

SELECT * FROM robots

WHERE name LIKE "%Robot%"; 

would yield all values that contain "Robot" inthe name.

Result:

id	name
1	Robot 2000
2	Champion Robot 2001
4	Turbo Robot 2002
5	Super Robot 2003
6	Super Turbo Robot 2004
7	Not A Robot
8	Unreleased Turbo Robot 2111

run a query that returns "Robot" followed by ayear between 2000 and 2099? (So 2015 is a valid value at the end, but 2123 isnot.) 

select * from robots

where robots.name like "%Robot 20__" ;

Result:

id	name
1	Robot 2000
2	Champion Robot 2001
4	Turbo Robot 2002
5	Super Robot 2003
6	Super Turbo Robot 2004

 Current tables:

friends_of_pickles

id	name	gender	species	height_cm
1	Dave	male	human	180
2	Mary	female	human	160
3	Fry	male	cat	30
4	Leela	female	cat	25
5	Odie	male	dog	40
6	Jumpy	male	dog	35
7	Sneakers	male	dog	55

            习题二十八、CASE

use a CASE statement to return certain values when certain scenariosare true. 

A CASE statement takes the following form: 

CASE WHEN *first thing is true* THEN *value1*
          WHEN *second thing is true*THEN *value2*
          ...
         ELSE *value for all othersituations* 
END 

 in order to return the number of legs foreach row in friends_of_pickles: 
SELECT *, 
           CASE WHEN species = 'human'THEN 2

 ELSE 4

END ASnum_legs 

FROM friends_of_pickles;

Result:

id	name	gender	species	height_cm	num_legs
1	Dave	male	human	180	2
2	Mary	female	human	160	2
3	Fry	male	cat	30	4
4	Leela	female	cat	25	4
5	Odie	male	dog	40	4
6	Jumpy	male	dog	35	4
7	Sneakers	male	dog	55	4

return the results with a column named sound that returns "talk" forhumans, "bark" for dogs, and "meow" for cats?

select * ,

case when species = 'human' then 'talk'

when species = 'dog' then 'bark'

when species = 'cat'  then 'meow'

end

as sound

from friends_of_pickles ;

Result:

id	name	gender	species	height_cm	sound
1	Dave	male	human	180	talk
2	Mary	female	human	160	talk
3	Fry	male	cat	30	meow
4	Leela	female	cat	25	meow
5	Odie	male	dog	40	bark
6	Jumpy	male	dog	35	bark
7	Sneakers	male	dog	55	bark

          Current tables:

robots

id	name	location
1	R2000 - Robot 2000	New City, NY
2	R2001 - Champion Robot 2001	Palo Alto, CA
3	D0001 - Dragon	New York City, NY
4	R2002 - Turbo Robot 2002	Spring Valley, NY
5	R2003 - Super Robot 2003	Nyack, NY
6	R2004 - Super Turbo Robot 2004	Tampa, FL
7	N0001 - Not A Robot	Seattle, WA
8	U2111 - Unreleased Turbo Robot 2111	Buffalo, NY

          习题二十九、SUBSTR

SUBSTR is used in this format: SUBSTR(column_name, index,number_of_characters) 

index is a number that denotes where you would start thesubstring. 1 would indicate the first character, 2 would indicated the secondcharacter, etc. The index could also be negative, which means you would countfrom the end of the string. -1 would denote the last character, -2 would denotethe 2nd to last character, etc.

number_of_characters is optional; if it is not included, thesubstring contains the "rest of the string". 

Here are some examples:
SUBSTR(name, 1, 5) is the first 5 characters of the name. 
SUBSTR(name, -4) is the last 4 characters of the name. 

SELECT * FROM robots

WHERE SUBSTR(name, -4) LIKE '20__'; 

is another way of returning all of the robotsthat have been released between 2000 and 2099.

Result:

id	name	location
1	R2000 - Robot 2000	New City, NY
2	R2001 - Champion Robot 2001	Palo Alto, CA
4	R2002 - Turbo Robot 2002	Spring Valley, NY
5	R2003 - Super Robot 2003	Nyack, NY
6	R2004 - Super Turbo Robot 2004	Tampa, FL

Note: In other versions of SQL, you could use RIGHT to do this.

return all of the robots that are located inNY?

SELECT *from robots

where substr (location,-2) like 'NY' ;

Result:

id	name	location
1	R2000 - Robot 2000	New City, NY
3	D0001 - Dragon	New York City, NY
4	R2002 - Turbo Robot 2002	Spring Valley, NY
5	R2003 - Super Robot 2003	Nyack, NY
8	U2111 - Unreleased Turbo Robot 2111	Buffalo, NY

         Current tables:

fighters

id	name	gun	sword	tank
1	US Marine	Colt 9mm SMG	Swiss Army Knife	M1A1 Abrams Tank
2	John Wilkes Booth	.44 caliber Derringer	null	null
3	Zorro	null	Sword of Zorro	null
4	Innocent Bystander	null	null	null

         习题三十、CIALESCE

COALESCE takes a list of columns, and returns thevalue of the first column that is not null. 

wanted to find the most powerful weapon that a combatanthas on hand. If value of gun is not null, that is the valuereturned. Otherwise, the value of sword is returned. Then you would run: 
SELECT name, COALESCE(gun, sword)

 as weapon

FROM fighters; 

Result:

name	weapon
US Marine	Colt 9mm SMG
John Wilkes Booth	.44 caliber Derringer
Zorro	Sword of Zorro
Innocent Bystander	null

a fighter's tank could count as a weapon, and it would takeprecedence over the gun and the sword. Could you find each fighter's weapon inthat scenario?

select name ,coalesce(tank,gun,sword)

          as weapon

         from fighters;

Result:

name	weapon
US Marine	M1A1 Abrams Tank
John Wilkes Booth	.44 caliber Derringer
Zorro	Sword of Zorro
Innocent Bystander	null