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遞歸查詢是指在SQL查詢中使用自我引用,以查詢層次結(jié)構(gòu)或遞歸關(guān)聯(lián)的數(shù)據(jù)。在PostgreSQL中,有兩種主要方法來實(shí)現(xiàn)遞歸查詢:使用公用表表達(dá)式(Common Table Expressions,CTEs)和遞歸的WITH子句。
WITH RECURSIVE employee_hierarchy AS (
SELECT id, name, parent_id
FROM employees
WHERE parent_id IS NULL
UNION ALL
SELECT e.id, e.name, e.parent_id
FROM employees e
JOIN employee_hierarchy eh ON e.parent_id = eh.id
)
SELECT * FROM employee_hierarchy;
WITH RECURSIVE employee_hierarchy AS (
SELECT id, name, parent_id
FROM employees
WHERE parent_id IS NULL
UNION ALL
SELECT e.id, e.name, e.parent_id
FROM employees e
JOIN employee_hierarchy eh ON e.parent_id = eh.id
)
SELECT * FROM employee_hierarchy;
PostgreSQL的擴(kuò)展函數(shù)和類型:
#include "postgres.h"
Datum add_numbers(PG_FUNCTION_ARGS) {
int32 a = PG_GETARG_INT32(0);
int32 b = PG_GETARG_INT32(1);
int32 result = a + b;
PG_RETURN_INT32(result);
}
#include "postgres.h"
typedef struct {
int32 id;
char name[50];
} Person;
typedef struct {
Person *persons;
int32 count;
} PersonList;
/* 創(chuàng)建Person類型的函數(shù) */
Person *create_person(int32 id, const char *name);
/* 創(chuàng)建PersonList類型的函數(shù) */
PersonList *create_person_list(int32 count);
總之,遞歸查詢是PostgreSQL中查詢層次結(jié)構(gòu)數(shù)據(jù)的有效方法,而擴(kuò)展函數(shù)和類型則為用戶提供了更多的自定義功能。
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