F.43. tablefunc
tablefunc 模組內含了回傳資料表(即多筆資料列)的各種函數。這些函數本身很有用,也可以用作設計回傳多筆資料列的 C 函數的範例。
F.43.1. Functions Provided
Table F.30 列出了 tablefunc 模組所提供的函數。
Table F.30. tablefunc
Functions
tablefunc
Functionsnormal_rand(int numvals, float8 mean, float8 stddev)
setof float8
Produces a set of normally distributed random values
crosstab(text sql)
setof record
Produces a “pivot table” containing row names plus N
value columns, where N
is determined by the row type specified in the calling query
crosstab
N
(text sql)
setof table_crosstab_
N
Produces a “pivot table” containing row names plus N
value columns. crosstab2
, crosstab3
, and crosstab4
are predefined, but you can create additional crosstab
N
functions as described below
crosstab(text source_sql, text category_sql)
setof record
Produces a “pivot table” with the value columns specified by a second query
crosstab(text sql, int N)
setof record
Obsolete version of crosstab(text)
. The parameter N
is now ignored, since the number of value columns is always determined by the calling query
connectby(text relname, text keyid_fld, text parent_keyid_fld [, text orderby_fld ], text start_with, int max_depth [, text branch_delim ])
setof record
Produces a representation of a hierarchical tree structure
F.43.1.1. Normal_rand
normal_rand
produces a set of normally distributed random values (Gaussian distribution).
numvals
is the number of values to be returned from the function. mean
is the mean of the normal distribution of values and stddev
is the standard deviation of the normal distribution of values.
For example, this call requests 1000 values with a mean of 5 and a standard deviation of 3:
F.43.1.2. Crosstab(Text)
The crosstab
function is used to produce “pivot” displays, wherein data is listed across the page rather than down. For example, we might have data like
which we wish to display like
The crosstab
function takes a text parameter that is a SQL query producing raw data formatted in the first way, and produces a table formatted in the second way.
The sql
parameter is a SQL statement that produces the source set of data. This statement must return one row_name
column, one category
column, and one value
column. N
is an obsolete parameter, ignored if supplied (formerly this had to match the number of output value columns, but now that is determined by the calling query).
For example, the provided query might produce a set something like:
The crosstab
function is declared to return setof record
, so the actual names and types of the output columns must be defined in the FROM
clause of the calling SELECT
statement, for example:
This example produces a set something like:
The FROM
clause must define the output as one row_name
column (of the same data type as the first result column of the SQL query) followed by N value
columns (all of the same data type as the third result column of the SQL query). You can set up as many output value columns as you wish. The names of the output columns are up to you.
The crosstab
function produces one output row for each consecutive group of input rows with the same row_name
value. It fills the output value
columns, left to right, with the value
fields from these rows. If there are fewer rows in a group than there are output value
columns, the extra output columns are filled with nulls; if there are more rows, the extra input rows are skipped.
In practice the SQL query should always specify ORDER BY 1,2
to ensure that the input rows are properly ordered, that is, values with the same row_name
are brought together and correctly ordered within the row. Notice that crosstab
itself does not pay any attention to the second column of the query result; it's just there to be ordered by, to control the order in which the third-column values appear across the page.
Here is a complete example:
You can avoid always having to write out a FROM
clause to define the output columns, by setting up a custom crosstab function that has the desired output row type wired into its definition. This is described in the next section. Another possibility is to embed the required FROM
clause in a view definition.
另請參閱 psql 中的 \crosstabview 指令,該指令提供的功能類似於 crosstab()。
F.43.1.3. CrosstabN(Text)
The crosstab
N
functions are examples of how to set up custom wrappers for the general crosstab
function, so that you need not write out column names and types in the calling SELECT
query. The tablefunc
module includes crosstab2
, crosstab3
, and crosstab4
, whose output row types are defined as
Thus, these functions can be used directly when the input query produces row_name
and value
columns of type text
, and you want 2, 3, or 4 output values columns. In all other ways they behave exactly as described above for the general crosstab
function.
For instance, the example given in the previous section would also work as
These functions are provided mostly for illustration purposes. You can create your own return types and functions based on the underlying crosstab()
function. There are two ways to do it:
Create a composite type describing the desired output columns, similar to the examples in
contrib/tablefunc/tablefunc--1.0.sql
. Then define a unique function name accepting onetext
parameter and returningsetof your_type_name
, but linking to the same underlyingcrosstab
C function. For example, if your source data produces row names that aretext
, and values that arefloat8
, and you want 5 value columns:Use
OUT
parameters to define the return type implicitly. The same example could also be done this way: