How do I use sqlite3 PRAGMA user_version in Objective-c?
I am trying to check the user_version of the sqlite DB. I have an admin tool to bump the version, but I don't understand the syntax of the pragma statement. I am expecting to test the value in an if开发者_运维问答-statement. Can someone provide a code sample? When I embed the pragma statement in my objective-c code, the compiler throws an error.
I figured it out with the inspiration from newtover, digging into FMDB and re-reading the sqlite3 documentation (it is still very vague in my opinion). This code returns the value that I bumped in the admin tool when I make notable changes to the schema that require migration.
-(int)queryUserVersion: (sqlite3*) db {
// get current database version of schema
static sqlite3_stmt *stmt_version;
int databaseVersion;
if(sqlite3_prepare_v2(db, "PRAGMA user_version;", -1, &stmt_version, NULL) == SQLITE_OK) {
while(sqlite3_step(stmt_version) == SQLITE_ROW) {
databaseVersion = sqlite3_column_int(stmt_version, 0);
NSLog(@"%s: version %d", __FUNCTION__, databaseVersion);
}
NSLog(@"%s: the databaseVersion is: %d", __FUNCTION__, databaseVersion);
} else {
NSLog(@"%s: ERROR Preparing: , %s", __FUNCTION__, sqlite3_errmsg(db) );
}
sqlite3_finalize(stmt_version);
return databaseVersion;
}
I have a similar method for the schema version where the sql-statement is changed to "PRAGMA schema_version;"
Pragma statements can not be used within other statements (there are no references to pragma-stmt from other statements).
But you can use the user_version value by making two requests: querying pragma and using the selected value as literal in the next query.
UPD: if you are interested in PRAGMA syntax, it is rather simple:
sqlite> pragma user_version=10;
sqlite> pragma user_version;
user_version
------------------------------
10
sqlite> pragma user_version='12.3.124';
sqlite> pragma user_version;
user_version
------------------------------
12
That is the result will come as a row with a single value.
If you are interested in a way to issue statements to SQLite in objective-c, try looking at neighbour questions: example. Unfortunatelly, I've never coded in objective-c.
If you are using FMDB wrapper (which is recommended if you don't want to deal with C handler interface of sqlite)
Use
[self.db setUserVersion:yourUserVersion]; // yourUserVersion is of uint32_t type
To read current user_version user
[self.db userVersion]; // returned value is of uint32_t type
Documentation:
http://ccgus.github.io/fmdb/html/Categories/FMDatabase+FMDatabaseAdditions.html
This answer is basically copy of this answer: https://stackoverflow.com/a/27807125/1364174
精彩评论