9.4. 字串函式及運算子

本節介紹了用於檢查和操作字串的函數和運算子。在這種情況下，字串包括 character、character varying 和 text 型別的值。除非另有說明，否則下面列出的所有函數都可以在這些型別上使用，但是請注意在使用 character 型別時自動空格填充的潛在影響。其中有一些函數還支援對於位元型別的處理。

SQL 定義了一些使用關鍵字而不是逗號分隔參數的字串函數。詳情請見 Table 9.9。PostgreSQL 還提供了使用一般函數呼叫的語法，這些功能的函數版本（請參見 Table 9.10）。

在 PostgreSQL 8.3 之前的版本中，由於存在從這些資料型別到文字的隱式強制轉換，這些函數也將默默接受幾種非字串資料型別的值。這些強制轉換已被刪除，因為它們經常引起令人驚訝的結果。但是，字串連接運算子（||）仍然接受非字串輸入，只要至少一個輸入為字串型別即可，如 Table 9.9 所示。對於其他情況，如果您需要複製以前的行為，請在查詢語句中明確加入型別轉換。

Table 9.9. SQL String Functions and Operators

Function

Return Type

Description

Example

Result

其他字串操作的可用函數，在 Table 9.10 中列出。其中一些用於內部實作的SQL標準字符串函數，則在 Table 9.9 中列出。

Table 9.10. Other String Functions

concat、concat_ws 和 format 函數是動態參數，因此可以將要連接或格式化的值以 VARIADIC 關鍵字標記的陣列（請參閱第 37.5.5 節）輸入。將陣列的元素視為函數的一個普通參數。如果動態參數陣列參數為 NULL，則 concat 和 concat_ws 回傳 NULL，但是 format 將 NULL 視為零元素陣列。

另請參閱第 9.20 節中的彙總函數 string_agg。

Table 9.11. Built-in Conversions

轉換名稱遵循標準的命名規則：來源編碼的正式名稱，所有非字母數字字元均用下底線代替，接在 _to___ 之後，然後是經過類似處理的目標編碼名稱。因此，名稱可能與習慣的編碼名稱有所不同。

9.4.1. `format`

The function format produces output formatted according to a format string, in a style similar to the C function sprintf.

format(formatstr text [, formatarg "any" [, ...] ])

formatstr is a format string that specifies how the result should be formatted. Text in the format string is copied directly to the result, except where format specifiers are used. Format specifiers act as placeholders in the string, defining how subsequent function arguments should be formatted and inserted into the result. Each formatarg argument is converted to text according to the usual output rules for its data type, and then formatted and inserted into the result string according to the format specifier(s).

Format specifiers are introduced by a % character and have the form

%[position][flags][width]type

where the component fields are:position (optional)

A string of the form n$ where n is the index of the argument to print. Index 1 means the first argument after formatstr. If the position is omitted, the default is to use the next argument in sequence.flags (optional)

Additional options controlling how the format specifier's output is formatted. Currently the only supported flag is a minus sign (-) which will cause the format specifier's output to be left-justified. This has no effect unless the width field is also specified.width (optional)

Specifies the minimum number of characters to use to display the format specifier's output. The output is padded on the left or right (depending on the - flag) with spaces as needed to fill the width. A too-small width does not cause truncation of the output, but is simply ignored. The width may be specified using any of the following: a positive integer; an asterisk (*) to use the next function argument as the width; or a string of the form *n$ to use the _n_th function argument as the width.

If the width comes from a function argument, that argument is consumed before the argument that is used for the format specifier's value. If the width argument is negative, the result is left aligned (as if the - flag had been specified) within a field of length abs(width).type (required)

The type of format conversion to use to produce the format specifier's output. The following types are supported:

s formats the argument value as a simple string. A null value is treated as an empty string.
I treats the argument value as an SQL identifier, double-quoting it if necessary. It is an error for the value to be null (equivalent to quote_ident).
L quotes the argument value as an SQL literal. A null value is displayed as the string NULL, without quotes (equivalent to quote_nullable).

In addition to the format specifiers described above, the special sequence %% may be used to output a literal % character.

Here are some examples of the basic format conversions:

SELECT format('Hello %s', 'World');
Result: Hello World

SELECT format('Testing %s, %s, %s, %%', 'one', 'two', 'three');
Result: Testing one, two, three, %

SELECT format('INSERT INTO %I VALUES(%L)', 'Foo bar', E'O\'Reilly');
Result: INSERT INTO "Foo bar" VALUES('O''Reilly')

SELECT format('INSERT INTO %I VALUES(%L)', 'locations', 'C:\Program Files');
Result: INSERT INTO locations VALUES('C:\Program Files')

Here are examples using width fields and the - flag:

SELECT format('|%10s|', 'foo');
Result: |       foo|

SELECT format('|%-10s|', 'foo');
Result: |foo       |

SELECT format('|%*s|', 10, 'foo');
Result: |       foo|

SELECT format('|%*s|', -10, 'foo');
Result: |foo       |

SELECT format('|%-*s|', 10, 'foo');
Result: |foo       |

SELECT format('|%-*s|', -10, 'foo');
Result: |foo       |

These examples show use of position fields:

SELECT format('Testing %3$s, %2$s, %1$s', 'one', 'two', 'three');
Result: Testing three, two, one

SELECT format('|%*2$s|', 'foo', 10, 'bar');
Result: |       bar|

SELECT format('|%1$*2$s|', 'foo', 10, 'bar');
Result: |       foo|

Unlike the standard C function sprintf, PostgreSQL's format function allows format specifiers with and without position fields to be mixed in the same format string. A format specifier without a position field always uses the next argument after the last argument consumed. In addition, the format function does not require all function arguments to be used in the format string. For example:

SELECT format('Testing %3$s, %2$s, %s', 'one', 'two', 'three');
Result: Testing three, two, three

The %I and %L format specifiers are particularly useful for safely constructing dynamic SQL statements. See Example 42.1.

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Table 9.9. SQL String Functions and Operators

Table 9.10. Other String Functions

Table 9.11. Built-in Conversions

9.4.1. format

Table 9.9. SQL String Functions and Operators

Table 9.10. Other String Functions

Table 9.11. Built-in Conversions

9.4.1. format

9.4.1. `format`

9.4.1. `format`