在 iOS11 的 Swift4 中，我想附加两个 SQLite 数据库文件(file1.db 和 file2.db)，以便对所有数据(即两个数据库)进行查询。
我已经使用以下代码在一个 SQLite 数据库中成功实现了一个 SQL 查询(..这里有一个名称查询的例子......):
func readOneDB() -> String? {
// create SQL-DB FilePath
do {
// create a documents-URL from filename
self.file_1_URL = try createSQLFilePath(fileName: "file1", fileExtension: "db")
}
catch {
return nil
}
let query =
"""
SELECT DISTINCT n.locations
FROM names n
WHERE n.name = "\(self.myName)"
"""
// Open SQLite database
var db: OpaquePointer? = nil
if sqlite3_open(self.file_1_URL?.absoluteString, &db) == SQLITE_OK {
var statement: OpaquePointer? = nil
// Run SELECT query from db
if sqlite3_prepare_v2(db, query, -1, &statement, nil) == SQLITE_OK {
// Loop through all results from query
while sqlite3_step(statement) == SQLITE_ROW {
let name = sqlite3_column_text(statement, 0)
if name != nil {
let versionString = String(cString:name!)
return nameString
} else {
print("name not found", terminator: "")
return nil
}
}
} else {
let errmsg = String(cString: sqlite3_errmsg(db))
print("error running query: \(errmsg)")
return nil
}
} else {
print("error opening database")
return nil
}
return nil
}
现在，我读到使用以下 sql-cmd 可以将两个 SQL 数据库附加在一起(以便对它们进行查询):ATTACH
事实上，我有以下代码不会抛出异常 - 但仍然不能 100% 工作
let attachStr = "ATTACH DATABASE \'\(self.file_2_URL!)\' AS SECOND"
if (sqlite3_exec(db, attachStr, nil, &statement, nil) == SQLITE_OK) {
print("Success!")
}
有“Success!”字样运行代码时。
当然，通过附加代码获得成功消息，我认为就是这样(并且 db-object 将两个数据库联合起来)
但实际上没有! : 不幸的是，查询仍然只带来第一个数据库(即 file1)的结果，没有附加的第二个 file2 的查询结果。
怎么了？
这是带有附件代码和查询的完整代码(见下文)。
为什么查询仍然只提供来自第一个数据库 (file1) 而不是来自两个数据库(即 file1 和 file2)的结果？？？？
我需要对代码进行哪些更改才能使其正常工作？？？？
func readTwoDBs() -> String? {
// create SQL-DB FilePath
do {
// create documents-URLs for two filenames
self.file_1_URL = try createSQLFilePath(fileName: "file1", fileExtension: "db")
self.file_2_URL = try createSQLFilePath(fileName: "file2", fileExtension: "db")
}
catch {
return nil
}
let query =
"""
SELECT DISTINCT n.locations
FROM names n
WHERE n.name = "\(self.myName)"
"""
// Open SQLite database
var db: OpaquePointer? = nil
if sqlite3_open(self.file_1_URL?.absoluteString, &db) == SQLITE_OK {
var statement: OpaquePointer? = nil
let attachStr = "ATTACH DATABASE \'\(self.file_2_URL!)\' AS SECOND"
if (sqlite3_exec(db, attachStr, nil, &statement, nil) == SQLITE_OK) {
print("Success!")
}
//// !!!!!!! Success is printed but the query still only works with file1 WHY ????????????????????
// Run SELECT query from db
statement = nil
if sqlite3_prepare_v2(db, query, -1, &statement, nil) == SQLITE_OK {
// Loop through all results from query
while sqlite3_step(statement) == SQLITE_ROW {
let name = sqlite3_column_text(statement, 0)
if name != nil {
let versionString = String(cString:name!)
return nameString
} else {
print("name not found", terminator: "")
return nil
}
}
} else {
let errmsg = String(cString: sqlite3_errmsg(db))
print("error running query: \(errmsg)")
return nil
}
} else {
print("error opening database")
return nil
}
return nil
}
最佳答案
在 Joakim 的帮助下，我想我找到了一个解决方案:
ATTACH-cmd 实际上意味着多个数据库共享同一个数据库连接。正如 Joakim 指出的那样，如果想从两个数据库文件中获取结果，仍然需要进行两次查询!
代码如下: (为了便于说明，该方法的返回值是一个元组，其中包含两个查询的两个结果(一个来自第一个数据库，第二个来自第二个数据库)
func readTwoDBs() -> (String, String)? {
var returnValue_1: String = ""
var returnValue_2: String = ""
// create SQL-DB FilePath
do {
// create documents-URLs for two filenames
self.file_1_URL = try createSQLFilePath(fileName: "file1", fileExtension: "db")
self.file_2_URL = try createSQLFilePath(fileName: "file2", fileExtension: "db")
}
catch {
return nil
}
let query_1 =
"""
SELECT DISTINCT n.locations
FROM names n
WHERE n.name = "\(self.myName)"
"""
// !! FOR QUERY_2 YOU NEED THE "MY_DB_TWO" NAME AND ITS TABLE IN THE FROM STATEMENT !!!!!!!
// !! THIS "MY_DB_TWO" IS GIVEN BY THE ATTACH-cmd FURTHER DOWN... !!!!!!!!
let query_2 =
"""
SELECT DISTINCT n.locations
FROM MY_DB_TWO.names n
WHERE n.name = "\(self.myName)"
"""
// Open SQLite database
var db: OpaquePointer? = nil
if sqlite3_open(self.file_1_URL?.absoluteString, &db) == SQLITE_OK {
var statement: OpaquePointer? = nil
let attachStr = "ATTACH DATABASE \'\(self.file_2_URL!)\' AS MY_DB_TWO"
if (sqlite3_exec(db, attachStr, nil, &statement, nil) == SQLITE_OK) {
print("Success!")
}
// Run SELECT query Nr1 from main.db
statement = nil
if sqlite3_prepare_v2(db, query_1, -1, &statement, nil) == SQLITE_OK {
// Loop through all results from query
while sqlite3_step(statement) == SQLITE_ROW {
let name = sqlite3_column_text(statement, 0)
if name != nil {
let nameString = String(cString:name!)
returnValue_1 = nameString
} else {
print("name not found", terminator: "")
returnValue_1 = ""
}
}
} else {
let errmsg = String(cString: sqlite3_errmsg(db))
print("error running query: \(errmsg)")
returnValue_1 = ""
}
// Run SELECT query Nr2 from MY_DB_TWO
statement = nil
if sqlite3_prepare_v2(db, query_2, -1, &statement, nil) == SQLITE_OK {
// Loop through all results from query
while sqlite3_step(statement) == SQLITE_ROW {
let name = sqlite3_column_text(statement, 0)
if name != nil {
let nameString = String(cString:name!)
returnValue_2 = nameString
} else {
print("name not found", terminator: "")
returnValue_2 = ""
}
}
} else {
let errmsg = String(cString: sqlite3_errmsg(db))
print("error running query: \(errmsg)")
returnValue_2 = ""
}
} else {
print("error opening database")
}
return (returnValue_1, returnValue_2)
}
关于sql - 附加两个 SQLite-DB 文件以在 Swift 中查询这两个文件，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/49440681/