SQL VISUAL QUICKSTART GUIDE- P16 ppt

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). 130 Chapter 5 Performing Arithmetic Operations Table 5.1 Arithmetic Operators Operator What It Does -expr Reverses the sign of expr +expr Leaves expr unchanged expr1 + expr2 Sums expr1 and expr2 expr1 – expr2 Subtracts expr2 from expr1 expr1 * expr2 Multiplies expr1 and expr2 expr1 / expr2 Divides expr1 by expr2 Listing 5.4 The negation operator changes the sign of a number. See Figure 5.4 for the result. SELECT title_id, -advance AS "Advance" FROM royalties; Listing title_id Advance T01 -10000.00 T02 -1000.00 T03 -15000.00 T04 -20000.00 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+expr2 to add, expr1-expr2 to subtract, expr1*expr2 to multiply, or expr1/expr2 to 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 131 Operators and Functions Performing Arithmetic Operations Listing 5.5 List the biographies by descending revenue (= price x sales). See Figure 5.5 for the result. SELECT title_id, price * sales AS "Revenue" 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 UPDATE ing rows, note that data types aren’t closed for some arithmetic operations. If you multiply or add two SMALLINT s, for example, the result might be greater than a SMALLINT column can hold. Similarly, dividing two INTEGER s doesn’t necessarily yield an INTEGER . ■ Sometimes DBMSs force 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 (an INTEGER ) 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). 132 Chapter 5 Performing Arithmetic Operations 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, pages/10 AS "pages/10", pages/10.0 AS "pages/10.0" 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 IN and 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. 133 Operators and Functions Determining the Order of Evaluation Table 5.2 Order of Evaluation (Highest to Lowest) Operator Description + , - Monadic identity, monadic negation * , / Multiplication, division + , - Addition, subtraction = , <> , < , <= , > , >= Comparison operators NOT Logical NOT AND Logical AND OR Logical OR 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", (2 + 3) * 4 AS "(2+3)*4", 6 / 2 * 3 AS "6/2*3", 6 / (2 * 3) AS "6/(2*3)"; 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’ is NULL . (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. 134 Chapter 5 Concatenating Strings with || 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 || in SELECT , WHERE , and ORDER BY clauses or anywhere an expres- sion is allowed. ■ You can concatenate hex and bit strings: B’0100’ || B’1011’ is B’01001011’ . ■ Listing 5.11 shows how to use || in a WHERE clause, 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 135 Operators and Functions Concatenating Strings with || 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 = 'Klee Hull'; 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 OR in MySQL by default. (Use PIPES_AS_CONCAT mode 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 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. 136 Chapter 5 Concatenating Strings with || Extracting a Substring with 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 section.) 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 length is 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). 137 Operators and Functions Extracting a Substring with SUBSTRING() 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 S. Buchman NY W. Heydemark CO C. Kells NY Figure 5.13 Result of Listing 5.13. ✔ Tips ■ You can use SUBSTRING() in SELECT , WHERE , and ORDER BY clauses 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’ . ■ In Microsoft Access, the sub- string function is Mid(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’ 138 Chapter 5 Extracting a Substring with SUBSTRING() 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 is SUBSTR(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. 139 Operators and Functions Extracting a Substring with SUBSTRING() . 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. 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. 5.11): au_fname + ‘ ‘ + au_lname ➝ = ‘Klee Hull’; In MySQL, the concatenation function is CONCAT() . The || operator is legal, but it means logical OR in MySQL by default. (Use PIPES_AS_CONCAT mode to