On 7/6/2014 10:47 PM, David G Johnston wrote:
madhu_d wroteHi, I am writing a C program to access a PostgreSQL database, where I add a column if it doesn't exists in the table or, update the column, if the column already exits. Please suggest how to work with the conditional statements. Thanks! N.B. I wrote the following: res = PQexec(conn, "IF COL_LENGTH('protein_sequence','comment') IS NULL"); PQclear(res); if(res) { res = PQexec(conn, "ALTER TABLE protein_sequence ADD comment VARCHAR(500)"); PQclear(res); } else { res = PQexec(conn, "UPDATE TABLE protein_sequence ADD comment VARCHAR(500)"); PQclear(res); } Is the code logically correct??Not by any logic that I find recognizable. It is also absolutely not syntactically correct. I have no clue why you think the updating of the column is conditional. I can understand needing to add a missing column before you can effect an update but that can and should be independent of the need to update the column. Neither "IF" nor "UPDATE TABLE ... ADD" are valid commands that you can issue directly via PQExec. The only valid commands are listed here: http://www.postgresql.org/docs/9.3/interactive/sql-commands.html Any other commands, of which conditionals are a subset, must be executed within the context of a DO command or user-defined function. In particular you should see if pl/pgsql can be made to accomplish that which you need. David J. -- View this message in context: http://postgresql.1045698.n5.nabble.com/conditional-IF-statements-in-postgresql-tp5810687p5810691.html Sent from the PostgreSQL - general mailing list archive at Nabble.com.
> Neither "IF" nor "UPDATE TABLE ... ADD" are valid commands that you can > issue directly via PQExec. I'm guessing that's Transact-SQL, which is microsoft only. You'll want to query out from information_schema: select * from information_schema.columns where table_name = 'protein_sequence' and column_name = 'comment'; Then fire off appropriate alter statements based on that. -Andy
