MySQL Procedure to synchronize the schema of 2 tables - mysql

Question

The goal is to add the missing columns when comparing the schemas of two tables in MySQL 5.5 (engine MyISAM). The argument p_table1 is the model table name from which p_table2 will be compared and "synchronized".
When it is called, nothing happens, no errors, no nothing. I've tried to log some variables but it hasn't worked either.
What could be wrong with the code?
CREATE PROCEDURE synchronize_tables(p_table1 VARCHAR(64), p_table2 VARCHAR(64), p_schema_name VARCHAR(64))
BEGIN
DECLARE v_done INT default false;
DECLARE v_actual_column_name VARCHAR(64);
DECLARE v_does_columns_exist INT default true;
DECLARE v_column_type LONGTEXT;
DECLARE v_column_default LONGTEXT;
DECLARE v_is_nullable VARCHAR(3);
DECLARE v_cur CURSOR FOR
SELECT column_name
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table1
AND table_schema = p_schema_name;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_done = TRUE;
DECLARE EXIT HANDLER FOR SQLEXCEPTION
BEGIN
OPEN v_cur;
read_loop: LOOP
FETCH v_cur INTO v_actual_column_name;
IF v_done THEN
LEAVE read_loop;
END IF;
SELECT count(*) INTO v_does_columns_exist
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table2
AND table_schema = p_schema_name
AND column_name = v_actual_column_name;
IF NOT v_does_columns_exist THEN
SELECT column_type, COLUMN_DEFAULT, IS_NULLABLE
INTO v_column_type, v_column_default, v_is_nullable
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table1
AND table_schema = p_schema_name
AND column_name = v_actual_column_name;
SET #stmt_text = CONCAT('ALTER TABLE ', p_schema_name, '.', p_table2,
' ADD COLUMN ', v_actual_column_name, ' ', v_column_type, ' ', IF(upper(v_is_nullable) = 'NO', 'NOT NULL', ''),
' DEFAULT ', v_column_default);
prepare v_stmt FROM #stmt_text;
execute v_stmt;
deallocate prepare v_stmt;
END IF;
END LOOP;
CLOSE v_cur;
END;
END

Answer 1

I found a couple of problems.
First is that most of your cursor code is inside an EXIT HANDLER FOR SQLEXCEPTION. This block is run only if an error occurs. So normally this block will never be run.
Second, you CONCAT() columns to form your ALTER TABLE statement, but one or more of the columns can be NULL. When you CONCAT() any string with a null, the result of the whole concat operation is NULL. So you have to make sure NULLs are defaulted to something non-NULL.
In my test, the column default is frequently NULL. We'd want this to become the keyword "NULL" in the ALTER TABLE statement. Also if the default is not NULL, you probably want to quote it, because an ordinary default value may be a string or a date, and you aren't quoting it. The solution: QUOTE() is a builtin function that quotes strings properly, and it even turns a NULL into the keyword "NULL".
Here's what I got to work:
CREATE PROCEDURE synchronize_tables(p_table1 VARCHAR(64),
p_table2 VARCHAR(64), p_schema_name VARCHAR(64))
BEGIN
DECLARE v_done INT default false;
DECLARE v_actual_column_name VARCHAR(64);
DECLARE v_does_columns_exist INT default true;
DECLARE v_column_type LONGTEXT;
DECLARE v_column_default LONGTEXT;
DECLARE v_is_nullable VARCHAR(3);
DECLARE v_cur CURSOR FOR
SELECT column_name
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table1
AND table_schema = p_schema_name;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_done = TRUE;
OPEN v_cur;
read_loop: LOOP
FETCH v_cur INTO v_actual_column_name;
IF v_done THEN
LEAVE read_loop;
END IF;
SELECT count(*) INTO v_does_columns_exist
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table2
AND table_schema = p_schema_name
AND column_name = v_actual_column_name;
IF NOT v_does_columns_exist THEN
SELECT column_type, COLUMN_DEFAULT, IS_NULLABLE
INTO v_column_type, v_column_default, v_is_nullable
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = p_table1
AND table_schema = p_schema_name
AND column_name = v_actual_column_name;
SET #stmt_text = CONCAT('ALTER TABLE ',
p_schema_name, '.', p_table2,
' ADD COLUMN ', v_actual_column_name, ' ',
v_column_type, ' ',
IF(upper(v_is_nullable) = 'NO', 'NOT NULL', ''),
' DEFAULT ', QUOTE(v_column_default));
PREPARE v_stmt FROM #stmt_text;
EXECUTE v_stmt;
DEALLOCATE prepare v_stmt;
END IF;
END LOOP;
CLOSE v_cur;
END
There are other problems with this approach:
Identifiers are not delimited.
It generates one ALTER TABLE for each missing column, even though one can add multiple columns in one ALTER.
It doesn't do the right thing when a column is NOT NULL but has no default.
However, I wouldn't do this with cursors anyway. There's an easier way:
CREATE PROCEDURE synchronize_tables(p_table1 VARCHAR(64),
p_table2 VARCHAR(64), p_schema_name VARCHAR(64))
BEGIN
SELECT CONCAT(
'ALTER TABLE `', C1.TABLE_SCHEMA, '`.`', p_table2, '` ',
GROUP_CONCAT(CONCAT(
'ADD COLUMN `', C1.COLUMN_NAME, '` ', C1.COLUMN_TYPE,
IF(C1.IS_NULLABLE='NO', ' NOT NULL ', ''),
IF(C1.COLUMN_DEFAULT IS NULL, '',
CONCAT(' DEFAULT ', QUOTE(C1.COLUMN_DEFAULT)))
) SEPARATOR ', '
)
) INTO #stmt_text
FROM INFORMATION_SCHEMA.COLUMNS AS C1
LEFT OUTER JOIN INFORMATION_SCHEMA.COLUMNS AS C2
ON C1.TABLE_SCHEMA=C2.TABLE_SCHEMA
AND C2.TABLE_NAME=p_table2
AND C1.COLUMN_NAME=C2.COLUMN_NAME
WHERE C1.TABLE_SCHEMA=p_schema_name AND C1.TABLE_NAME=p_table1
AND C2.TABLE_SCHEMA IS NULL;
PREPARE v_stmt FROM #stmt_text;
EXECUTE v_stmt;
DEALLOCATE prepare v_stmt;
END
This makes one ALTER TABLE to add all columns. It delimits identifiers. It handles NOT NULL better.
But even my solution still has problems:
Causes an error if you try to add a NOT NULL column with no DEFAULT to a populated table2.
Doesn't notice extra columns that exist in the second table but not the first table.
Doesn't synchronize constraints, indexes, triggers, procedures, or views.
It's a very complex task to sync database structure completely. I suggest that trying to use MySQL's stored procedure language for this is making a hard task even harder.
MySQL's implementation of stored procedures sucks.
Poor documentation.
No support for packages.
No standard procedures or rich function library.
No real debugger exists (some tools try, but they're faking it).
No support for persisting compiled procedures. Procedures are recompiled by every session that uses them.
I often recommend to developers who are accustomed to using procedures in Oracle or Microsoft SQL Server, to stay away from MySQL stored procedures.