SQL VISUAL QUICKSTART GUIDE- P16 ppt

Performing ArithmeticOperations A monadic or unary arithmetic operator performs a mathematical operation on a single numeric operand to produce a result.. A dyadic or binary arithmetic o

Performing Arithmetic

Operations

A monadic (or unary) arithmetic operator

performs a mathematical operation on a

single numeric operand to produce a result

The -(negation) operator changes the sign

of its operand, and the not-very-useful +

(identity) operator leaves its operand

unchanged A dyadic (or binary) arithmetic

operator performs a mathematical operation

on two numeric operands to produce a result

These operators include the usual ones: +

(addition), -(subtraction), *(multiplication),

and/(division) Table 5.1 lists SQL’s

arith-metic operators (expr is a numeric expression).

To change the sign of a number:

◆ Type -expr

expr is a numeric expression (Listing 5.4

and Figure 5.4).

Table 5.1

Arithmetic Operators

-expr Reverses the sign of expr

+expr Leaves expr unchanged

Listing 5.4 The negation operator changes the sign of

a number See Figure 5.4 for the result.

SELECT title_id,

FROM royalties;

Listing

title_id Advance - -T01 -10000.00

T02 -1000.00

T03 -15000.00

T05 -100000.00

T06 -20000.00

T07 -1000000.00

T08 0.00

T09 0.00

T10 NULL

T11 -100000.00

T12 -50000.00

T13 -20000.00

Figure 5.4 Result of Listing 5.4 Note that zero has no

sign (is neither positive nor negative).

To add, subtract, multiply, or divide:

◆ Type expr1+expr2to add, expr1-expr2to subtract, expr1*expr2to multiply, or

expr1/expr2to divide

expr1 and expr2 are numeric expressions

(Listing 5.5 and Figure 5.5).

✔ Tips

■ The result of any arithmetic operation that involves a null is null

■ If you use multiple operators in a single expression, you may need to use paren-theses to control the calculation order; see

“Determining the Order of Evaluation”

later in this chapter

■ If you mix numeric data types in an arithmetic expression, your DBMS

con-verts, or coerces, all the numbers to the

data type of the expression’s most com-plex operand and returns the result in this type This conversion process is

called promotion If you add an INTEGER

and a FLOAT, for example, the DBMS con-verts the integer to a float, adds the numbers, and returns the sum as a float

In some cases, you must convert a data type to another data type explicitly; see

“Converting Data Types with CAST()” later in this chapter

continues on next page

Listing 5.5 List the biographies by descending

revenue (= price x sales) See Figure 5.5 for the result.

SELECT title_id,

FROM titles

WHERE type = 'biography'

ORDER BY price * sales DESC;

Listing

title_id Revenue

-

-T07 35929790.00

T12 1299012.99

T06 225834.00

T10 NULL

Figure 5.5 Result of Listing 5.5.

Other Operators and Functions

All DBMSs provide plenty of operators

and functions in addition to those

defined in the SQL standard (or covered

in this book) In fact, the standard is

play-ing catch-up—many of the functions

introduced in the latest standard have

existed for years in DBMSs The earlier

standards were so anemic that they left

SQL weaker than a desktop calculator If

you search your DBMS documentation

for operators and functions, you’ll find

mathematical, statistical, financial,

scien-tific, trigonometric, conversion, string,

datetime, bitwise, system, metadata,

security, and other entries

■ If you’re writing a database application or

UPDATEing rows, note that data types aren’t

closed for some arithmetic operations

If you multiply or add two SMALLINTs, for

example, the result might be greater than

aSMALLINTcolumn can hold Similarly,

dividing two INTEGERs doesn’t necessarily

yield an INTEGER

mathe-matical closure, so be careful when dividing integers by integers If an

integer dividend is divided by an integer

divisor, the result may be an integer that

has any fractional part of the result

trun-cated You might expect the two derived

columns in Listing 5.6 to contain the

same values, because the column pages

(anINTEGER) is divided by two equal

constants: 10 (an integer) and 10.0 (a

float) Microsoft Access, Oracle, and

MySQL return the result you’d expect

(Figure 5.6a), but Microsoft SQL Server,

DB2, and PostgreSQL truncate the result

of an integer division (Figure 5.6b).

Listing 5.6 This query’s first derived column

divides pages by the integer constant 10, and the second derived column divides pages by the floating-point constant 10.0 In the result, you’d expect identical values to be in both derived columns See Figures 5.6a and 5.6b for the results.

SELECT title_id, pages,

FROM titles;

Listing

title_id pages pages/10 pages/10.0

- - -

-T01 107 10.7 10.7

T02 14 1.4 1.4

T03 1226 122.6 122.6

T04 510 51.0 51.0

T05 201 20.1 20.1

T06 473 47.3 47.3

T07 333 33.3 33.3

T08 86 8.6 8.6

T09 22 2.2 2.2

T10 NULL NULL NULL

T11 826 82.6 82.6

T12 507 50.7 50.7

T13 802 80.2 80.2

Figure 5.6a Result of Listing 5.6 for Microsoft Access,

Oracle, and MySQL Dividing two integers yields a

floating-point number (as you’d expect).

title_id pages pages/10 pages/10.0 - - - -T01 107 10 10.7

T02 14 1 1.4

T03 1226 122 122.6

T04 510 51 51.0

T05 201 20 20.1

T06 473 47 47.3

T07 333 33 33.3

T08 86 8 8.6

T09 22 2 2.2

T10 NULL NULL NULL

T11 826 82 82.6

T12 507 50 50.7

T13 802 80 80.2

Figure 5.6b Result of Listing 5.6 for Microsoft SQL

Server, DB2, and PostgreSQL Dividing two integers yields an integer; the fractional part of the result is discarded (not as you’d expect).

Determining the Order

of Evaluation

Precedence determines the priority of

vari-ous operators when more than one operator

is used in an expression Operators with higher precedence are evaluated first

Arithmetic operators (+,-,*, and so on) have higher precedence than comparison opera-tors (<,=,>, and so on), which have higher precedence than logical operators (NOT,AND,

OR), so the expression

a or b * c >= d

is equivalent to

a or ((b * c) >= d)

Operators with lower precedence are less

binding than those with higher precedence.

Table 5.2 lists operator precedences from

most to least binding Operators in the same row have equal precedence

Associativity determines the order of

evaluation in an expression when adjacent operators have equal precedence SQL uses left-to-right associativity

You don’t need to memorize all this information You can use parentheses to override precedence and associativity rules

(Listing 5.7 and Figure 5.7).

✔ Tips

■ Table 5.2 is incomplete; it omits

some standard (such as INand

EXISTS) and nonstandard (DBMS-specific) operators To determine the complete order of evaluation that your DBMS uses, search your DBMS documentation for

precedence.

To run Listing 5.7 in Oracle, add the

clause FROM DUAL To run it in DB2, add

the clause FROM SYSIBM.SYSDUMMY1 See the DBMS Tip in “Creating Derived Columns” earlier in this chapter

Table 5.2

Order of Evaluation (Highest to Lowest)

+ , - Monadic identity, monadic negation

* , / Multiplication, division

+ , - Addition, subtraction

= , <> , < , <= , > , >= Comparison operators

NOT Logical NOT

AND Logical AND

Listing 5.7 The first and second columns show how

to use parentheses to override precedence rules

The third and fourth columns show how to use

parentheses to override associativity rules See

Figure 5.7 for the result.

SELECT 2 + 3 * 4 AS "2+3*4",

Listing

2+3*4 (2+3)*4 6/2*3 6/(2*3)

- -

-14 20 9 1

Figure 5.7 Result of Listing 5.7.

■ It’s good programming style to add

parentheses (even when they’re

unneces-sary) to complex expressions to ensure

your intended evaluation order and make

code more portable and easier to read

Concatenating Strings

with ||

Use the operator ||to combine, or

concate-nate, strings The operator’s important

char-acteristics are:

◆ The operator ||is two consecutive

vertical-bar, or pipe, characters

◆ Concatenation doesn’t add a space

between strings

◆ ||, a dyadic operator, combines two

strings into a single string: ‘formal’

|| ‘dehyde’is‘formaldehyde’

◆ You can chain concatenations to

com-bine multiple strings into a single string:

‘a’ || ‘b’ || ‘c’ || ‘d’is‘abcd’

◆ Concatenation with an empty string (‘’)

leaves a string unchanged: ‘a’ || ‘’ ||

‘b’is‘ab’

◆ The result of any concatenation

opera-tion that involves a null is null: ‘a’ ||

NULL || ‘b’isNULL (But see the

Oracle exception in the DBMS Tip in

this section.)

◆ To concatenate a string and a nonstring

(such as a numeric or datetime value),

you must convert the nonstring to a

string if your DBMS doesn’t convert it

implicitly; see “Converting Data Types

with CAST()” later in this chapter

Listing 5.8 List the authors’ first and last names,

concatenated into a single column and sorted by last name/first name See Figure 5.8 for the result.

SELECT au_fname || ' ' || au_lname

AS "Author name"

FROM authors ORDER BY au_lname ASC, au_fname ASC;

Listing

Author name -Sarah Buchman Wendy Heydemark Hallie Hull Klee Hull Christian Kells Kellsey Paddy O'Furniture

Figure 5.8 Result of Listing 5.8.

Listing 5.9 List biography sales by descending sales

order Here, I need to convert sales from an integer to

a string See Figure 5.9 for the result.

SELECT CAST(sales AS CHAR(7))

|| ' copies sold of title '

|| title_id

AS "Biography sales"

FROM titles WHERE type = 'biography' AND sales IS NOT NULL ORDER BY sales DESC;

Listing

Biography sales

-1500200 copies sold of title T07

100001 copies sold of title T12

11320 copies sold of title T06

Figure 5.9 Result of Listing 5.9.

To concatenate strings:

◆ Type:

string1 || string2

string1 and string2 are the strings to be

combined Each operand is a string expression such as a column that con-tains character strings, a string literal,

or the result of an operation or function

that returns a string (Listings 5.8 through

5.11, Figures 5.8 through 5.11).

✔ Tips

■ You can use ||inSELECT,WHERE, and

ORDER BYclauses or anywhere an expres-sion is allowed

■ You can concatenate hex and bit strings:

B’0100’ || B’1011’isB’01001011’

■ Listing 5.11 shows how to use ||in a

WHEREclause, but it’s actually bad SQL.

The efficient way to express the clause is:

WHERE au_fname = ‘Klee’

AND au_lname = ‘Hull’

■ You can use the TRIM()function to remove unwanted spaces from concate-nated strings Recall from “Character String Types” in Chapter 3 that CHAR

values are padded with trailing spaces, sometimes creating long, ugly stretches

of spaces in concatenated strings TRIM()

will remove the extra space in front of

the name Kellsey in Figure 5.8, for

exam-ple; see “Trimming Characters with

TRIM()” later in this chapter

continues on next page

Listing 5.10 List biographies by descending publication

date Here, I need to convert pubdate from a datetime

to a string See Figure 5.10 for the result.

SELECT 'Title '

|| title_id

|| ' published on '

|| CAST(pubdate AS CHAR(10))

AS "Biography publication dates"

FROM titles

WHERE type = 'biography'

AND pubdate IS NOT NULL

ORDER BY pubdate DESC;

Listing

Biography publication dates

-Title T12 published on 2000-08-31

Title T06 published on 2000-07-31

Title T07 published on 1999-10-01

Figure 5.10 Result of Listing 5.10.

Listing 5.11 List all the authors named Klee Hull See

Figure 5.11 for the result.

SELECT au_id, au_fname, au_lname

FROM authors

WHERE au_fname || ' ' || au_lname

Listing

au_id au_fname au_lname

-

-A04 Klee Hull

Figure 5.11 Result of Listing 5.11.

■ In Microsoft Access, the

con-catenation operator is +, and the conversion function is Format(string)

To run Listings 5.8 through 5.11, change

the concatenation and conversion

expressions to (Listing 5.8):

au_fname + ‘ ‘ + au_lname

and (Listing 5.9):

Format(sales)

➝ + ‘ copies sold of title ‘

➝ + title_id

and (Listing 5.10):

‘Title ‘

➝ + title_id

➝ + ‘ published on ‘

➝ + Format(pubdate)

and (Listing 5.11):

au_fname + ‘ ‘ + au_lname

➝ = ‘Klee Hull’;

In Microsoft SQL Server, the

concate-nation operator is + To run Listings 5.8

through 5.11, change the concatenation

expressions to (Listing 5.8):

au_fname + ‘ ‘ + au_lname

and (Listing 5.9):

CAST(sales AS CHAR(7))

➝ + ‘ copies sold of title ‘

➝ + title_id

and (Listing 5.10):

‘Title ‘

➝ + title_id

➝ + ‘ published on ‘

➝ + CAST(pubdate AS CHAR(10))

and (Listing 5.11):

au_fname + ‘ ‘ + au_lname

➝ = ‘Klee Hull’;

In MySQL, the concatenation function is

CONCAT() The ||operator is legal, but it means logical ORin MySQL by default (Use

PIPES_AS_CONCATmode to treat ||as a string-concatenation operator rather than

as a synonym for OR.)CONCAT()takes any number of arguments and converts non-strings to non-strings as necessary (so CAST()

isn’t needed) To run Listings 5.8 through 5.11, change the concatenation expressions

to (Listing 5.8):

CONCAT(au_fname, ‘ ‘, au_lname)

and (Listing 5.9):

CONCAT(sales,

➝ ’ copies sold of title ‘,

➝ title_id)

and (Listing 5.10):

CONCAT(‘Title ‘,

➝ title_id,

➝ ’ published on ‘,

➝ pubdate)

and (Listing 5.11):

CONCAT(au_fname, ‘ ‘, au_lname)

➝ = ‘Klee Hull’;

Oracle treats an empty string as null: ‘a’

|| NULL || ‘b’returns ‘ab’ See the DBMS Tip in “Nulls” in Chapter 3

Oracle, MySQL, and PostgreSQL convert

nonstrings to strings implicitly in concat-enations; Listings 5.9 and 5.10 still will run

on these DBMSs if you omit CAST() Search

your DBMS documentation for concatenation

or conversion.

Oracle and DB2 also support the CONCAT()

function

Extracting a Substring

Use the function SUBSTRING()to extract part

of a string The function’s important charac-teristics are:

◆ A substring is any sequence of

contigu-ous characters from the source string, including an empty string or the entire source string itself

◆ SUBSTRING()extracts part of a string starting at a specified position and continuing for a specified number of characters

◆ A substring of an empty string is an empty string

◆ If any argument is null, SUBSTRING()

returns null (But see the Oracle

excep-tion in the DBMS Tip in this secexcep-tion.)

To extract a substring:

◆ Type:

SUBSTRING(string FROM start [FOR length])

string is the source string from which to extract the substring string is a string

expression such as a column that con-tains character strings, a string literal, or the result of an operation or function that

returns a string start is an integer that

specifies where the substring begins, and

length is an integer that specifies the

length of the substring (the number of

characters to return) start starts counting

at 1 If FOR lengthis omitted, SUBSTRING()

returns all the characters from start to

the end of string (Listings 5.12 through

5.14, Figures 5.12 through 5.14).

Listing 5.12 Split the publisher IDs into alphabetic

and numeric parts The alphabetic part of a publisher

ID is the first character, and the remaining characters

are the numeric part See Figure 5.12 for the result.

SELECT pub_id,

SUBSTRING(pub_id FROM 1 FOR 1)

AS "Alpha part",

SUBSTRING(pub_id FROM 2)

AS "Num part"

FROM publishers;

Listing

pub_id Alpha part Num part

-

-P01 P 01

P02 P 02

P03 P 03

P04 P 04

Figure 5.12 Result of Listing 5.12 Listing 5.13 List the first initial and last name of the authors from New York State and Colorado See Figure 5.13 for the result SELECT SUBSTRING(au_fname FROM 1 FOR 1) || ' ' || au_lname AS "Author name", state FROM authors WHERE state IN ('NY', 'CO'); Listing Author name state

Figure 5.13 Result of Listing 5.13.

✔ Tips

■ You can use SUBSTRING()inSELECT,

WHERE, and ORDER BYclauses or anywhere

an expression is allowed

■ You can extract substrings from hex and

bit strings: SUBSTRING(B’01001011’ FROM

5 FOR 4)returns B’1011’

sub-string function isMid(string, start [,length]) Use +to concatenate

strings To run Listings 5.12 through 5.14,

change the substring expressions to

(Listing 5.12):

Mid(pub_id, 1, 1)

Mid(pub_id, 2)

and (Listing 5.13):

Mid(au_fname, 1, 1) + ‘ ‘ + au_lname

and (Listing 5.14):

Mid(phone, 1, 3)=’415’

In Microsoft SQL Server, the substring

function is SUBSTRING(string, start,

length) Use +to concatenate strings To

run Listings 5.12 through 5.14, change the

substring expressions to (Listing 5.12):

SUBSTRING(pub_id, 1, 1)

SUBSTRING(pub_id, 2, LEN(pub_id)-1)

and (Listing 5.13):

SUBSTRING(au_fname, 1, 1)

➝ + ‘ ‘

➝ + au_lname

and (Listing 5.14):

SUBSTRING(phone, 1, 3)=’415’

Listing 5.14 List the authors whose area code is 415.

See Figure 5.14 for the result.

SELECT au_fname, au_lname, phone FROM authors

WHERE SUBSTRING(phone FROM 1 FOR 3)='415';

Listing

au_fname au_lname phone - - -Hallie Hull 415- 549-4278 Klee Hull 415- 549-4278

Figure 5.14 Result of Listing 5.14.

In Oracle and DB2, the substring function

isSUBSTR(string, start [,length]) To run

Listings 5.12 through 5.14, change the

sub-string expressions to (Listing 5.12):

SUBSTR(pub_id, 1, 1)

SUBSTR(pub_id, 2)

and (Listing 5.13):

SUBSTR(au_fname, 1, 1)

➝ || ‘ ‘

➝ || au_lname

and (Listing 5.14):

SUBSTR(phone, 1, 3)=’415’

In MySQL, use CONCAT()to run Listing 5.13 (see “Concatenating Strings with ||” earlier

in this chapter) Change the concatenation

expression to:

CONCAT(

➝ SUBSTRING(au_fname FROM 1 FOR 1),

➝ ’ ‘,

➝ au_lname)

Oracle treats an empty string as null:

SUBSTR(NULL, 1, 2)returns ‘’ See the

DBMS Tip in “Nulls” in Chapter 3

Your DBMS implicitly might constrain start and length arguments that are too small or

too large to sensible values The substring

function silently may replace a negative start with 1 or a too-long length with the length of string, for example Search your DBMS docu-mentation for substring or substr.

MySQL and PostgreSQL also support the

SUBSTR(string, start, length)form of the

substring function