# 9.1第九章

## 函数和操作(Functions and Operators)

### 逻辑操作符(Logical Operators)

The usual logical operators are available:

AND OR NOT SQL uses a three-valued Boolean logic where the null value represents "unknown". Observe the following truth tables:

 a b a AND b a OR b TRUE TRUE TRUE TRUE TRUE FALSE FALSE TRUE TRUE NULL NULL TRUE FALSE FALSE FALSE FALSE FALSE NULL FALSE NULL NULL NULL NULL NULL
 a NOT a TRUE FALSE FALSE TRUE NULL NULL

The operators AND and OR are commutative, that is, you can switch the left and right operand without affecting the

result. But see Section 4.2.14 for more information about the order of evaluation of subexpressions. The usual comparison operators are available, shown in Table 9-1.

### 比较操作符号（Comparison Operators）

The usual comparison operators are available, shown in Table 9-1. Table 9-1. Comparison Operators'

 Operator Description < less than > greater than <= less than or equal to >= greater than or equal to = equal <> or != not equal

Note: The != operator is converted to <> in the parser stage. It is not possible to implement != and <>

operators that do different things.

Comparison operators are available for all relevant data types. All comparison operators are binary operators that

return values of type boolean; expressions like 1 < 2 < 3 are not valid (because there is no < operator to compare

a Boolean value with 3).

In addition to the comparison operators, the special BETWEEN construct is available:

```a BETWEEN x AND y
```

is equivalent to

```a >= x AND a <= y
```

Notice that BETWEEN treats the endpoint values as included in the range. NOT BETWEEN does the opposite comparison:

```a NOT BETWEEN x AND y
```

is equivalent to

```a < x OR a > y
```

BETWEEN SYMMETRIC is the same as BETWEEN except there is no requirement that the argument to the left of AND be

less than or equal to the argument on the right. If it is not, those two arguments are automatically swapped, so

that a nonempty range is always implied.

To check whether a value is or is not null, use the constructs:

```expression IS NULL
expression IS NOT NULL
or the equivalent, but nonstandard, constructs:

expression ISNULL
expression NOTNULL
```

Do not write expression = NULL because NULL is not "equal to" NULL. (The null value represents an unknown value,

and it is not known whether two unknown values are equal.) This behavior conforms to the SQL standard.

Tip: Some applications might expect that expression = NULL returns true if expression evaluates to the null

value. It is highly recommended that these applications be modified to comply with the SQL standard. However, if

that cannot be done the transform_null_equals configuration variable is available. If it is enabled, PostgreSQL

will convert x = NULL clauses to x IS NULL.

Note: If the expression is row-valued, then IS NULL is true when the row expression itself is null or when

all the row's fields are null, while IS NOT NULL is true when the row expression itself is non-null and all the

row's fields are non-null. Because of this behavior, IS NULL and IS NOT NULL do not always return inverse results

for row-valued expressions, i.e., a row-valued expression that contains both NULL and non-null values will return

false for both tests. This definition conforms to the SQL standard, and is a change from the inconsistent behavior

exhibited by PostgreSQL versions prior to 8.2.

Ordinary comparison operators yield null (signifying "unknown"), not true or false, when either input is null. For

example, 7 = NULL yields null. When this behavior is not suitable, use the IS [ NOT ] DISTINCT FROM constructs:

expression IS DISTINCT FROM expression expression IS NOT DISTINCT FROM expression For non-null inputs, IS DISTINCT FROM is the same as the <> operator. However, if both inputs are null it returns

false, and if only one input is null it returns true. Similarly, IS NOT DISTINCT FROM is identical to = for non-

null inputs, but it returns true when both inputs are null, and false when only one input is null. Thus, these

constructs effectively act as though null were a normal data value, rather than "unknown".

Boolean values can also be tested using the constructs

```expression IS TRUE
expression IS NOT TRUE
expression IS FALSE
expression IS NOT FALSE
expression IS UNKNOWN
expression IS NOT UNKNOWN
```

These will always return true or false, never a null value, even when the operand is null. A null input is treated

as the logical value "unknown". Notice that IS UNKNOWN and IS NOT UNKNOWN are effectively the same as IS NULL and

IS NOT NULL, respectively, except that the input expression must be of Boolean type.

### 数学函数和操作符(Mathematical Functions and Operators)

Mathematical operators are provided for many PostgreSQL types. For types without standard mathematical conventions

(e.g., date/time types) we describe the actual behavior in subsequent sections.

Table 9-2 shows the available mathematical operators.

Table 9-2. Mathematical Operators

The bitwise operators work only on integral data types, whereas the others are available for all numeric data

types. The bitwise operators are also available for the bit string types bit and bit varying, as shown in Table 9-

10.

Table 9-3 shows the available mathematical functions. In the table, dp indicates double precision. Many of these

functions are provided in multiple forms with different argument types. Except where noted, any given form of a

function returns the same data type as its argument. The functions working with double precision data are mostly

implemented on top of the host system's C library; accuracy and behavior in boundary cases can therefore vary

depending on the host system. Table 9-3. Mathematical Functions

 Function Return Type Description Example Result abs(x) (same as input) absolute value abs(-17.4) 17.4 cbrt(dp) dp cube root cbrt(27.0) 3 ceil(dp or numeric) (same as input) smallest integer not less than argument ceil(-42.8) -42 ceiling(dp or numeric) (same as input) smallest integer not less than argument (alias for ceil) ceiling(-95.3) -95 degrees(dp) dp radians to degrees degrees(0.5) 28.6478897565412 div(y numeric, x numeric) numeric integer quotient of y/x div(9,4) 2 exp(dp or numeric) (same as input) exponential exp(1.0) 2.71828182845905 floor(dp or numeric) (same as input) largest integer not greater than argument floor(-42.8) ln(dp or numeric) (same as input) natural logarithm ln(2.0) 0.693147180559945 log(dp or numeric) (same as input) base 10 logarithm log(100.0) 2 log(b numeric, x numeric) numeric logarithm to base b log(2.0, 64.0) 6.0000000000 mod(y, x) (same as argument types) remainder of y/x mod(9,4) 1 pi() dp "π" constant pi() 3.14159265358979 power(a dp, b dp) dp a raised to the power of b power(9.0, 3.0) 729 power(a numeric, b numeric) numeric a raised to the power of b power(9.0, 3.0) 729 radians(dp) dp degrees to radians radians(45.0) 0.785398163397448 random() dp random value in the range 0.0 <= x < 1.0 random() round(dp or numeric) (same as input) round to nearest integer round(42.4) 42 round(v numeric, s int) numeric round to s decimal places round(42.4382, 2) 42.44 setseed(dp) void set seed for subsequent random() calls (value between -1.0 and 1.0, inclusive) setseed(0.54823) sign(dp or numeric) (same as input) sign of the argument (-1, 0, +1) sign(-8.4) -1 sqrt(dp or numeric) (same as input) square root sqrt(2.0) 1.4142135623731 trunc(dp or numeric) (same as input) truncate toward zero trunc(42.8) 42 trunc(v numeric, s int) numeric truncate to s decimal places trunc(42.4382, 2) 42.43 width_bucket(op numeric, b1 numeric, b2 numeric, count int) int return the bucket to which operand would be assigned in an equidepth histogram with count buckets, in the range b1 to b2 width_bucket(5.35, 0.024, 10.06, 5) 3 width_bucket(op dp, b1 dp, b2 dp, count int) int return the bucket to which operand would be assigned in an equidepth histogram with count buckets, in the range b1 to b2 width_bucket(5.35, 0.024, 10.06, 5) 3

Finally, Table 9-4 shows the available trigonometric functions. All trigonometric functions take arguments and

return values of type double precision. Trigonometric functions arguments are expressed in radians. Inverse

functions return values are expressed in radians. See unit transformation functions radians() and degrees() above.

Table 9-4. Trigonometric Functions

 Function Description acos(x) inverse cosine asin(x) inverse sine atan(x) inverse tangent atan2(y, x) inverse tangent of y/x cos(x) cosine cot(x) cotangent sin(x) sine tan(x) tangent

### String Functions and Operators

This section describes functions and operators for examining and manipulating string values. Strings in this context include values of the types character, character varying, and text. Unless otherwise noted, all of the functions listed below work on all of these types, but be wary of potential effects of automatic space-padding when using the character type. Some functions also exist natively for the bit-string types.

SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 9-5. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 9-6).

Note: Before PostgreSQL 8.3, these functions would silently accept values of several non-string data types as well, due to the presence of implicit coercions from those data types to text. Those coercions have been removed because they frequently caused surprising behaviors. However, the string concatenation operator (||) still accepts non-string input, so long as at least one input is of a string type, as shown in Table 9-5. For other cases, insert an explicit coercion to text if you need to duplicate the previous behavior. Table 9-5. SQL String Functions and Operators

Additional string manipulation functions are available and are listed in Table 9-6. Some of them are used internally to implement the SQL-standard string functions listed in Table 9-5.

Table 9-6. Other String Functions

Table 9-7. Built-in Conversions

 Conversion Name [a] Source Encoding Destination Encoding ascii_to_mic SQL_ASCII MULE_INTERNAL ascii_to_utf8 SQL_ASCII UTF8 big5_to_euc_tw BIG5 EUC_TW big5_to_mic BIG5 MULE_INTERNAL big5_to_utf8 BIG5 UTF8 euc_cn_to_mic EUC_CN MULE_INTERNAL euc_cn_to_utf8 EUC_CN UTF8 euc_jp_to_mic EUC_JP MULE_INTERNAL euc_jp_to_sjis EUC_JP SJIS euc_jp_to_utf8 EUC_JP UTF8 euc_kr_to_mic EUC_KR MULE_INTERNAL euc_kr_to_utf8 EUC_KR UTF8 euc_tw_to_big5 EUC_TW BIG5 euc_tw_to_mic EUC_TW MULE_INTERNAL euc_tw_to_utf8 EUC_TW UTF8 gb18030_to_utf8 GB18030 UTF8 gbk_to_utf8 GBK UTF8 iso_8859_10_to_utf8 LATIN6 UTF8 iso_8859_13_to_utf8 LATIN7 UTF8 iso_8859_14_to_utf8 LATIN8 UTF8 iso_8859_15_to_utf8 LATIN9 UTF8 iso_8859_16_to_utf8 LATIN10 UTF8 iso_8859_1_to_mic LATIN1 MULE_INTERNAL iso_8859_1_to_utf8 LATIN1 UTF8 iso_8859_2_to_mic LATIN2 MULE_INTERNAL iso_8859_2_to_utf8 LATIN2 UTF8 iso_8859_2_to_windows_1250 LATIN2 WIN1250 iso_8859_3_to_mic LATIN3 MULE_INTERNAL iso_8859_3_to_utf8 LATIN3 UTF8 iso_8859_4_to_mic LATIN4 MULE_INTERNAL iso_8859_4_to_utf8 LATIN4 UTF8 iso_8859_5_to_koi8_r ISO_8859_5 KOI8R iso_8859_5_to_mic ISO_8859_5 MULE_INTERNAL iso_8859_5_to_utf8 ISO_8859_5 UTF8 iso_8859_5_to_windows_1251 ISO_8859_5 WIN1251 iso_8859_5_to_windows_866 ISO_8859_5 WIN866 iso_8859_6_to_utf8 ISO_8859_6 UTF8 iso_8859_7_to_utf8 ISO_8859_7 UTF8 iso_8859_8_to_utf8 ISO_8859_8 UTF8 iso_8859_9_to_utf8 LATIN5 UTF8 johab_to_utf8 JOHAB UTF8 koi8_r_to_iso_8859_5 KOI8R ISO_8859_5 koi8_r_to_mic KOI8R MULE_INTERNAL koi8_r_to_utf8 KOI8R UTF8 koi8_r_to_windows_1251 KOI8R WIN1251 koi8_r_to_windows_866 KOI8R WIN866 koi8_u_to_utf8 KOI8U UTF8 mic_to_ascii MULE_INTERNAL SQL_ASCII mic_to_big5 MULE_INTERNAL BIG5 mic_to_euc_cn MULE_INTERNAL EUC_CN mic_to_euc_jp MULE_INTERNAL EUC_JP mic_to_euc_kr MULE_INTERNAL EUC_KR mic_to_euc_tw MULE_INTERNAL EUC_TW mic_to_iso_8859_1 MULE_INTERNAL LATIN1 mic_to_iso_8859_2 MULE_INTERNAL LATIN2 mic_to_iso_8859_3 MULE_INTERNAL LATIN3 mic_to_iso_8859_4 MULE_INTERNAL LATIN4 mic_to_iso_8859_5 MULE_INTERNAL ISO_8859_5 mic_to_koi8_r MULE_INTERNAL KOI8R mic_to_sjis MULE_INTERNAL SJIS mic_to_windows_1250 MULE_INTERNAL WIN1250 mic_to_windows_1251 MULE_INTERNAL WIN1251 mic_to_windows_866 MULE_INTERNAL WIN866 sjis_to_euc_jp SJIS EUC_JP sjis_to_mic SJIS MULE_INTERNAL sjis_to_utf8 SJIS UTF8 tcvn_to_utf8 WIN1258 UTF8 uhc_to_utf8 UHC UTF8 utf8_to_ascii UTF8 SQL_ASCII utf8_to_big5 UTF8 BIG5 utf8_to_euc_cn UTF8 EUC_CN utf8_to_euc_jp UTF8 EUC_JP utf8_to_euc_kr UTF8 EUC_KR utf8_to_euc_tw UTF8 EUC_TW utf8_to_gb18030 UTF8 GB18030 utf8_to_gbk UTF8 GBK utf8_to_iso_8859_1 UTF8 LATIN1 utf8_to_iso_8859_10 UTF8 LATIN6 utf8_to_iso_8859_13 UTF8 LATIN7 utf8_to_iso_8859_14 UTF8 LATIN8 utf8_to_iso_8859_15 UTF8 LATIN9 utf8_to_iso_8859_16 UTF8 LATIN10 utf8_to_iso_8859_2 UTF8 LATIN2 utf8_to_iso_8859_3 UTF8 LATIN3 utf8_to_iso_8859_4 UTF8 LATIN4 utf8_to_iso_8859_5 UTF8 ISO_8859_5 utf8_to_iso_8859_6 UTF8 ISO_8859_6 utf8_to_iso_8859_7 UTF8 ISO_8859_7 utf8_to_iso_8859_8 UTF8 ISO_8859_8 utf8_to_iso_8859_9 UTF8 LATIN5 utf8_to_johab UTF8 JOHAB utf8_to_koi8_r UTF8 KOI8R utf8_to_koi8_u UTF8 KOI8U utf8_to_sjis UTF8 SJIS utf8_to_tcvn UTF8 WIN1258 utf8_to_uhc UTF8 UHC utf8_to_windows_1250 UTF8 WIN1250 utf8_to_windows_1251 UTF8 WIN1251 utf8_to_windows_1252 UTF8 WIN1252 utf8_to_windows_1253 UTF8 WIN1253 utf8_to_windows_1254 UTF8 WIN1254 utf8_to_windows_1255 UTF8 WIN1255 utf8_to_windows_1256 UTF8 WIN1256 utf8_to_windows_1257 UTF8 WIN1257 utf8_to_windows_866 UTF8 WIN866 utf8_to_windows_874 UTF8 WIN874 windows_1250_to_iso_8859_2 WIN1250 LATIN2 windows_1250_to_mic WIN1250 MULE_INTERNAL windows_1250_to_utf8 WIN1250 UTF8 windows_1251_to_iso_8859_5 WIN1251 ISO_8859_5 windows_1251_to_koi8_r WIN1251 KOI8R windows_1251_to_mic WIN1251 MULE_INTERNAL windows_1251_to_utf8 WIN1251 UTF8 windows_1251_to_windows_866 WIN1251 WIN866 windows_1252_to_utf8 WIN1252 UTF8 windows_1256_to_utf8 WIN1256 UTF8 windows_866_to_iso_8859_5 WIN866 ISO_8859_5 windows_866_to_koi8_r WIN866 KOI8R windows_866_to_mic WIN866 MULE_INTERNAL windows_866_to_utf8 WIN866 UTF8 windows_866_to_windows_1251 WIN866 WIN windows_874_to_utf8 WIN874 UTF8 euc_jis_2004_to_utf8 EUC_JIS_2004 UTF8 ut8_to_euc_jis_2004 UTF8 EUC_JIS_ 2004 shift_jis_2004_to_utf8 SHIFT_JIS_2004 UTF8 ut8_to_shift_jis_2004 UTF8 SHIFT_JIS_2004 euc_jis_2004_to_shift_jis_2004 EUC_JIS_2004 SHIFT_JIS_2004 shift_jis_2004_to_euc_jis_2004 SHIFT_JIS_2004 EUC_JIS_2004

Notes: a. The conversion names follow a standard naming scheme: The official name of the source encoding with all non-alphanumeric characters replaced by underscores, followed by _to_, followed by the similarly processed destination encoding name. Therefore, the names might deviate from the customary encoding names.

### Binary String Functions and Operators

This section describes functions and operators for examining and manipulating values of type bytea.

SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 9-8. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 9-9).

Table 9-8. SQL Binary String Functions and Operators

Additional binary string manipulation functions are available and are listed in Table 9-9. Some of them are used internally to implement the SQL-standard string functions listed in Table 9-8.

Table 9-9. Other Binary String Functions get_byte and set_byte number the first byte of a binary string as byte 0. get_bit and set_bit number bits from the right within each byte; for example bit 0 is the least significant bit of the first byte, and bit 15 is the most significant bit of the second byte.